Any animal vets on here?

Hey all, so we have a female pit who is 13. On Tuesday she was panting extremely hard out of nowhere, wouldn’t eat, drink, was walking a little wobbly and didn’t have any interest in her favorite activities, so I called the vet and got her in. Weighing her, she lost less than 6lbs but still - she lost. They did an x-ray, as seen below, and noticed a mass in her in abdomen(white circle on lower part of stomach image). They said they can’t say if it’s cancerous or not without an ultra sound and biopsy but assumed it was such due to her not wanting to eat, drink, etc. Unfortunately, the visit and ultra sound/biopsy would’ve been over $1,200 so we did without. They put her on pain meds and anti-inflammatory meds. Since we started the meds, (prednisone and gabapentin), she’s seemed a little better. She’s eating, drinking a lot more and moving more than she did on Tuesday. My question is, is there a way to know if it’s cancerous besides an ultra sound? Would you think she is she acting better due to the meds or was Tuesday just a weird fluke? I’m too scared to lose my girl. 😭

Pic of her Tuesday ❤️