will cardio lower resting heart rate at the gym?

[bashmasterflex]

Hi everyone. Recently I got sick with mono and lost a ton of weight and am very upset about it. Thankfully I had a quick recovery apparently (3 weeks ha) and now Im back at the gym and feeling healthy. However, I noticed that the same preworkout supps I used to take before I got sick will give me a serious heart rate, palpitations, anxiety, high BP etc. Today I had to leave 10 mins into the gym because how bad I felt, I felt like i was about to get a heart attack. Im good now though. I have not done anything in those 3 weeks... might it be that I have had no conditioning? I also did not drink any caffiene during those 3 weeks either but honestly after a long day if I really need the kick of a preworkout. If I dont take it im asleep, if I do, my heart is pounding. Any input would be appreciated. Thanks