Why burps from Iron Pill?

I took the tender iron for about 6 months & always got the burps.  They are indeed gross!  I just switched to Ferret's Ferrous Fumarate - vitalady also sells it - NO BURPS, not even a hint.  The tender iron's are carbonyl iron, which is elemental iron (same thing, different name).  The ferrous fumarate is tender iron.  I had to take 12 tender irons to get 300 mg of elemental iron.  I take 3 of the Ferret's to get 318mg of elemental iron . . . & NO BURPS.  It costs $3.50 more per month.  Totally worth it to not get the burps, not have to chew them up, & not have to take nearly as many.  I've been on the Ferret's for one week now.