What Causes Vomiting During Bowel Movement While Treating Anal Cancer?

I am 2 years out from anal cancer - 2 5-day rounds of chemo & 30 days of radiation. For the past 5-6 months, I have been vomiting while having a bowel movement (which could be described as soft-serve ice cream). There is usually a moment or two of acute pain (as if my stool is passing through a difficult area) followed by the taste in my mouth that precludes vomiting. Many times I've not eaten anything and it's dry heave. I just saw my internist and she is prescribing an antacid. I've never had acid reflux. this is happening 3-4 times a month. Any feedback?