Possible ruptured abscess?

My mom and I share a house and she noticed earlier that her cat, Roo, has a large sore next to her butt. It has what looks like pus in it and is about the size of a pinto bean. It doesn't seem to bother her too much as she's still moving around the house and acts normal aside from sometimes licking the area. Of course, I tend to freak out and jump to the worst possible thing and fear it could be cancer, though there is no weight loss and everything else about her seems to be fine at this point, other than the fact that she hasn't pestered my mom as much as she usually does. I doused it with some colloidal silver after cleaning it a bit with the colloidal silver on a cotton ball. I'm torn on whether or not to rush her to the vet and stressing her out if it's something that can be dealt with here at home. I'm just one who really hates putting cats through something unnecessary, you know? I also had three cats at the vet last week, all for different reasons, and it seems that has stressed out the ones that didn't have to go. I dunno, I know stress can cause weird issues. I've had a lot of cats throughout my life and very fortunately, I haven't seen cancer in any thus far, so I have no idea what I should even look for. I've seen abscesses, some really bad ones, on outdoor cats that were worse than this that did just fine. That was back when we allowed cats outdoors, but we no longer do that. All of our cats are indoor only, so she hasn't been exposed to any neighborhood cats or strays. I'm just hoping someone might have some ideas and suggestions. I'm going to be keeping an eye on her for the rest of the day, checking the area, and also cleaning it with the colloidal silver. I'm debating a trip to the vet tomorrow morning or Saturday morning if nothing else pops up. If she stops eating, becomes lethargic, or starts vomiting, we will definitely rush her straight in to be checked.

When my cat had an issue with her anal gland, we honestly didn't notice until it ruptured, then she was constantly licking at it and THAT's when we noticed it was oozing gunk.  We took her in and they cleaned it all up (squeezing all the puss out and washed it out with something like a water pik ), then she got antibiotics and an ointment for us to put on it, even though we knew she would promptly lick it off.  We were also advised to apply warm compresses to the area a few times a day.  When we left, the area was about the size of a quarter after all the squeezing and cleaning of it.  

After 5 days it closed up nicely, just from doing the above.  She actually seemed to enjoy the warm compresses

I think the colloidal silver is good to use if the wound is "open", but you said it hasn't festered.  Is there any leakage? 

From the sounds of it, I would NOT be thinking cancer, I would definitely be thinking anal gland issue

Thank you for your reply. There does seem to be pus or some other type of whitish gunk leaking from it. It's open a good bit, doesn't look like it's bleeding very much, if at all. It's strange that I've had so many cats over the years and this is totally new to me, haha. I didn't even know about anal glands until a few years ago when a friend of mine was talking about her dog having an issue with hers. Is there anything in particular that a cat does, any signs before anal gland problems get this bad? She's currently sitting on the ottoman giving my mom the food look, so it doesn't appear that she's in too much pain. She ate her wet food earlier without any problems. Since it's most likely her anal gland, we'll go ahead and take her to the vet in the morning. I'll definitely feel better knowing that they've cleaned it and hopefully give her medicine for it. I swear, I think my cats are having a competition on who can get the highest vet bill, hahaha. I'm glad that you think it's most likely not cancer, though. I weighed the cats earlier and she has lost a little weight, but not an alarming amount. However, I must admit that she's overweight. We have them all on grain free food and the ones that are overweight seem to be losing some of the excess ounces, which is why the weight loss on some of them isn't scaring me to death. 

As i mentioned, we didn't know anything was wrong with OUR cat until it ruptured, and I was aware that some cats needed to have their anal glands expressed manually, etc.  This was the first, and hopefully last time we ever have this type of problem in our household of cats AND dogs.  I'm pretty sure it happened 2 - 3 years ago, and she was 12 or 13 when it happened.  When you're at the Vet, maybe ask if there are signs you can look for in the future.  I think "scooting" may be a sign they need to be expressed, unless they have a "hanger on"

The people who have cats with consistent problems cannot seem to pinpoint any particular issue that cause it.  Not food, not specific breeds, not specific health conditions, etc.

Ah, okay. Yeah, I'll ask the vet about what signs there might be. It seems like I'm learning a lot of new things about cats just from the ones we have now. Thank you very much for taking the time to answer my questions and explain things to me. I don't feel anywhere near as frightened as I was when we first found it, so you've helped me out a great deal!