Hi, hopefully it is nothing but stress or caffeine ;)! But if it takes going to the ER to find out then do it. I would see if you can get them to run some different tests. My EKG's are fine yet I have some heart related problems. EKG's don't show everything. Perhaps if you go to the ER they will do chest xrays and an echocardiogram at least. It may give you more peace of mind to have these done. Congrats on the pregnancy!!
PS. I quit caffeine completely after finding out I have cardiomyopathy. While I wasn't told to, it just makes me feel better and I know that my palpitations aren't caused by caffeine. I used to be a real cola addict!
I know caffeine can effect you in a variety of ways as can stress. It does seem (to me at least) that Doctors think everyone is stressed and needs to stop drinking coffee and soda. Personally I think the stress usually follows the symptoms and doesn't cause them. Just my opinion.
I don't think going to the ER would be overreacting. You are expecting a child (Congrats!) and just to ensure your health and the safety of the Baby it wouldn't hurt to get it checked out.
I pray that you would have peace as you deal with this.