I have no thyroid, I get terrible stomach muscle spasms, but not internal spasms...probably my hormone levels aren't right. Tell your doctor, may be related, but then again, it is good to be thorough.
You only have (had, in your case) one thyroid gland, which has 2 lobes and an isthmus.
I don't know that your stomach issue is related to the thyroid, so yes, I'd agree that you should see a gastroenterologist. If there's something else wrong, maybe it can be fixed before further damage is done.
Do you know if you have Hashimoto's Thyroiditis or Graves Disease? Can you post your latest thyroid blood test results, along with the reference ranges, which vary from lab to lab, so must come from your own report.