I Recently Had A Heart Attack. I Had A Stent

I recently had a heart attack. I had a stent put in. After cardiac rehab, I began going to a gym regularly. I have notice my resting heart rate has been going down for a"normal low" of 49 to about 42 BPM. I was thinking that I may be getting dehydrated. I find I drink nor coffee as a substitute for the cigarette I used to have. Any thoughts?