im a 16 year old girl and have just been put onto ADHD med's (Concerta)... Every once in a while I can feel my heart beating really fast, and it's accompanied by a bit of breathlessness. It happens most often when i'm just sitting or lying down, and I don't know what's causing it. All my medicine says is that it might cause dizzyness. I'm not sure how long it's been going on for, but I noticed it more recently, withing the past couple of months. I asked my mom about it, but she said she didn't think it was too much of an issue. can anyone help?