Oh bless you! Agree it sounds like your stomach just isn't used to what it's digesting!
I don't have any experience of that this pregnancy (very much a risk I might throw up from said act.. 7 months of nausea and counting..!) but I do find my heartburn to be totally random. Yes, the usual suspects generally make it worse but one day, it'll suddenly be really bad from eating a plain ham sandwich or some cereal, which in theory should be pretty unacidic and bland! Some days, I can eat say a tomato pasta and have no heartburn after it when you'd think it'd be a killer! I hate heartburn, it's so irritating and makes you feel rubbish but at the same time, is so common I feel bad complaining about it!
Hope some antacids settle it down for you! Plus, it's good protein for baby