It is my understanding that calcium citrate is easier to absorb than carbonate. I don't know if Nature's Bounty has more than one, but a quick look showed carbonate. There are some fillers there too that maybe you are reacting to?
36yr old mother of 3 Moderate Pancolitis 1/3/17. Hosp 1 week on Solumedrol, Delzicol. Apriso, pred, then switched to Balsazide 750 mg, and Uceris 9 mg 3 wks. Minor flare in August, flare since Oct 20. Acute pancreatitis from Balsalazide hosp. 2 days, Started Humira 12/15/17, reaction, Entyvio 1/12/18. 40 mg of Pred, still waiting for a change!