Would your vet be willing to prescribe you a small amount of tramadol? It's non-NSAID so doesn't promote bleeding. See if the tramadol stops the panting. If it does, it may be that she is panting due to discomfort (in which case, see if your vet will keep her on the tramadol). If it doesn't stop the panting, then either she's just getting hot or the tumor is interfering with the air flow. Our Priscilla panted more at the end, more from an air flow problem than from pain.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."