Hello, thanks for your question today! I am Dr. Drew, and I am a licensed veterinarian. I'll be happy to help you in any way I can.
Hypothyroidism would be an unlikely explanation for this sort of situation,
but it is possible, in theory, that a mast cell tumor could cause some intermittent discomfort, as these types of tumors can release Histamine, which can induce inflammation.
Usually there's signs of swelling and/or discoloration of the skin around a mast cell tumor, when histamine has been released, but not always.
My most likely explanation, however, would be some type of nerve or muscle issues, as you've alluded to,
that sudden, sharp appearance of discomfort, would be more consistent with something neurologically related or controlled.
and I wouldn't typically expect a mast cell tumor to be associated with that.
A pinched nerve, or inflammation around a nerve, could cause such signs,
It would be reasonable to try a course of anti inflammatory medicine and muscle relaxants, to see if the problem is resolved,
if not, then it's up to you whether or not to pursue additional testing, such as blood tests, and then an MRI of the spine.