Am i just over worried?

A month or so ago, i went in to the ER for Cocaine overdose. I had been freebasing/snorting several times a month for three months prior to that. Im 18, started using drugs at 14. In the beginning i was doing Robitussin nearly every week for a year and a half or so. Started smoking marijuana on a daily basis, drinking all the time, experimenting with halluccinagens. I have pains in my chest under my left bottomost rib. but the doc said i just pulled some muscles in my chest. sometimes it hurts BAD though. In the past I've ingested lethal amounts of DXM, the drug in cough syrup that produces the high. A week ago, i called 911 because i was in fear that i was going to have a heart attack. They did blood tests and said my heart and kidneys are ok. But what worries me is that sometimes I feel like im going to die, just drop dead and no one could tell why. Ill feel my heart beat increase randomnly w exursion, and palpitations occur once in a while. IS THIS DRUG WITHDRAWLS?? Thank u all!!