Loose stool and it stinks

Freya, my precious little kitty, is my two year old, nearly three year old. I've had her since February but she's already my world and everything. She's been acting fine lately, running around the house crazily and trying to get into the bathroom when I'm in there(she obnoxiously has taken up the habit of scratching doors, no idea where she picked this up from). Well, I was in the litter box room, when I noticed her go in and it smelled horrifying, I checked the box and oh my gosh, it was a bit liquidy(but solid). She's acting normal, running around like a bat out of hell, so she doesn't seem to be in any discomfort(she had a bladder issue after we first got her and she would not stop crying), so I came to check here to make sure it's not anything to worry about before asking my mother to make a vet appointment.

Currently she is eating Nutrish premium food for cats, and although she wont touch it, nine lives wet food(which I have been trying to get my mother NOT to buy because Freya wont touch it, and it's just wasting food). Is her not eating the wet food causing the loose stool? Or could it be something else? Thank you guys for your help.

A loose and smelly stool can be caused by so many things that it's really worth taking her to the vet if it keeps up more than a day.  Has she been tested for all the usual parasites and pathogens?  

As you suspect, the problem might also be diet related - the wrong diet often results in very smelly stools.  Not eating the wet food would not cause the smell, to answer your question.   You're right to urge your mother to purchase a higher quality canned food than 9 Lives, though.  Fancy Feast pates are pretty decent and less expensive, especially on sale, than many of the high quality canned foods.  

It may come down to you keeping a food diary to identify any food allergies that Freya might have.