How old is Lilah?
I think no matter what, you should definitely have your kitty seen by the Vet.
I surely do NOT mean to scare you; remember, I'm only sharing my particular experience....there can be lumps that are benign (fatty lumps called lipomas), for instance. There are cysts, abscesses, etc..........so this could be nothing but........it might be something more significant.
My cat Taco, back the beginning of April (he was 15 yrs ago at the time) developed a pretty firm (but freely movable) pea-sized lump to his right shoulder blade area. although it was freely movable, it felt like it was connected in a way (to muscle). It was rather firm feeling. I didn't like it. To the Vet. He's so passive, he just said "watch it for 2-4 weeks" then we can decide if we remove it. In 2 weeks it was double the size, the size of a grape. This lump came out of nowhere because he's diabetic, I give him insulin twice a day and there's no way I'd have previously missed/not felt it. It was very fast growing.
I had it removed near the end of April (I did not want to bother with him doing a needle biopsy; the risk with that is that if it's a malignant tumor, the needle is disrupting the cells and spread can occur). I don't think Vet was aggressive enough in removing it (and surrounding tissue). The diagnosis: Osteosarcoma of the soft tissue. The pathologist report indicated that it was likely "vaccine associated sarcoma" due to the location of it (also known as "VAS"). It returned 4 weeks later, to the day. This time he did extensive removal. Taco recovered very well both times.
Sadly in Sept, I felt a new lump, just to the side of the first spot. it was pea sized and freely movable................it's now half the size of an egg and hard as a rock. I opted for no more surgery -- I know it's an aggressive tumor and due to how much tissue and muscle was removed last time, there's nothing left to remove there now. He's since developed a new one on this side.
He's fine, usual self, no pain, no impact to mobility, I have him on some supplements to boost his immune system, I belong to the Feline VAS group at Yahoo for support and questions.............
In our case, I think the Vet was wrong to suggest I just sit and wait. I should have demanded he remove it immediately. I also don't think he took wide enough margins the first time. aggressive removal of surrounding tissue is imperative. Even better if the person doing the surgery is a Vet surgeon or someone a little more experienced in tumor removal than your average general Vet.
I opted to not go with radiation or chemo (which most cats do very well and this, along with the surgery, can greatly reduce the chances of lump return)....because Taco is old and so are his kidneys, plus we'd have had to travel 45 minutes each way, to the Vet hospital that does this........he can't even handle a 5 minute ride to local vet across town without throwing up and peeing from the stress. No thanks.
Anyway, from all of this............this very well may NOT be what your kitty has, I'm not saying it is..............just sharing my experience. My vet diddled around too much.
There are lots of cats on the VAS Group I go to who got this when younger, and had the chemo and/or radiation along with surgery and they're still going strong years later with no recurrence.
Just don't let your Vet be passive and tell you "oh it's nothing." Vaccine Associated Sarcoma is sadly very common. I stopped vaccinating Taco 13 yrs ago and from what I've read, it can take that long before it develops (they believe the preservative in vaccines years ago caused an inflammation reaction that leads to VAS and have since started giving vaccinations near the legs instead of the scruff, for this very reason, so that if a sarcoma occurs, they can amputate the leg and cat can go on to live a great life).
Please keep us posted.
Taco also used to get fatty lumps years ago, which of course were nothing..........