So sorry to hear that there's a health concern with Ruby.
Your vet will probably need to do more investigations,xrays,blood tests and perhaps an ultrasound or biopsy.
None of these are cheap I'm afraid.
It may be inflammation or infection too,it's just impossible to say.
Ideally,you'll need to arrange a visit with your vet,have a look at the xrays with them and discuss what needs to be done and an estimated cost.
The fact that she is poorly suggests it needs to be investigated quickly and give her pain relief if she's uncomfortable.
My Daisy had liver cancer at the end of last year,investigations as described above were approx £1800.
Sharing my sofa with Holly, Ivy,Lilly and Hazy.. and never forgetting our beautiful Daisy who reached the bridge too soon.