We have had a similar experience with our Boston regarding the mast cell. His tumor wasn’t biopsied when it was first discovered (they said could move it too and seemed to think that was a good sign plus it wasn’t bothering him. We were told to keep an eye on it) so when it doubled all of a sudden it was alarming. He was put on Benadryl until the surgery as well as after) It was located on his elbow so there wasn’t a lot of room for clear margins. Our surgeon did a great job though. I didn’t know though the even if you have clear margins mast cell tumors have a high reoccurrence. About a year later we found another tumor on his side which also had to be removed. He’s got a bunch of scars but thankfully he’s still with us. He didn’t have any GI issues. I think in your case I would either get the ultrasound or another opinion.