Hi Heather There is several possibilities here: 1) Fluid-filled cyst - these can be drained which is sometimes curative, but they can recurr and surgical excision might be necessary. 2) Abscess - These are caused by an injury, often puncture wounds, that causes bacterial infection and pus to collect under the skin. They are usually painful though. 3) Tumour - Can be benign or malignant, as it is movable and squishy it does sound like a fatty lump (benign) but there is no way to diagnose a tumour 100% without taking a biopsy and send off for histology. If your vet suspects a tumour he/she might suggest to have it removed, which would be the safest thing to do in case it is a malignant growth. If you do not want her to go through a general anaesthetic you might want to wait for a couple of weeks to see if it increases in size or gets more painful, in which case you should definitely have it removed surgically. I hope this answers your question Torunn Beurling DrMedVet