33 weeks and tachycardia?

Hello everyone, i'm 24 and 33 weeks pregnant. Last night I went to the OB because I just felt i couldn't breath and was very dizzy. My heart rate, resting, was 134-139. After they monitored baby they moved me to the ER. There the doctor treated me and sent me home saying I had tachycardia. This is not the first time this has happen just the first time it has scared me that bad. I'm calling my OBGYN in the morning. I'm just wondering what your experience with tachycardia is and any information you may have good or bad. when should i worry and not worry? thank you
also I have been having contraction/Braxton hicks and low back aches. could this affect it as well?