Antihistamines really hurt my stomach, took claritin (loratadine) 10mg this time now my stomach is killing me, why does it happen w anthistamines? Is it Dangerous?

Antihistamines. Are known to cause severe hunger pains in some individuals, the newer ones as desloratadine or levo cetirizine might not cause this side effect. However, I see that you have LBBB, you need to be cautious using antihistamines, unless really needed, may cause heart rate problem in some patients, discuss with your doctor, best wishes.