Body Has Completely Changed Itself!!!

Hey all, I was hoping someone could answer this question I have. Why is it that my body has suddenly changed so much that I cannot take any form of medicine otc? It started with decongestants, we finally figured out that they cause my heart to skip and so I can't take decongestants. But I've been having problems sleeping lately so I started taking just an otc sleeping aid. Well eventually my heart started skipping again. Why is it that medicines suddenly affect me so much? Please answer.

Oh and another question if someone can anwer,
my heart does still skip beats every now and then and they say i have sinus arrythmia, is that a symptom of it?