I've had a similar issue. My HR gets high when I work out or do cardio. When I jog it gets scary at times. I've had it get as high as 236bpm on a recumbent bike. I've been to a Cardiologist and had a stress test done (he shut it down too early b/c it was climbing too rapidly). The nurse had me hooked up to the monitor when I was sitting in the chair waiting on the doctor, he came in and asked me to step up on the tredmill. As soon as I stood up the nurse blurted out "holy sh!t!". My HR went from like 65bpm to I think like 120bpm just from standing up. They also did an EKG and echogram (I think it was). He said that my heart was fine that there was another issue that was triggering it to beat fast. He thought that either I had a hyperthyroid or low iron and sent me to an endocrinologist. I had blood work done and the endocrinologist said that everything looks fine. So now I have paid maore money than I'd like for Dr. Suess to send me to Dr. Jekyll and still not have an answer.
If your HR conserns you I would go to a Dr. and get it checked out, just pray that you don't go to the same doctors that I did.