I have an elevated heart rate. What are the possible causes?!

I have an elevated heart rate. What are the possible causes?

I'm athletic, young and blood pressure is fine. I noticed though that for the past few months I've been having an average of 90 beats per min. I used to have a resting heart rate of 55. I've had several physical checkups lateley and doctors dont seem to be worried. My lifestyle and diet havent changed much since before too. I also dont take coffee or any stimulating drugs. Should i worry?

3 weeks ago
Nope didnt have an EKG. Thanks, i will pursue for an explanation

There are many different possible reasons. A fever,stress, thyroid, problem possibly something hormonal. That is a big rise, but you are not really in danger unless your resting is over 100 which means you are tacycardic. Keep mentioning it to your doctor and tell him that you are starting to get worried and if he did some tests it would at least put your mind at rest. Only other thing is that if the rhythm is fine they wont be worried.
Good Luck