It happened to me when I was in my 6th month. My experience was truly scary. My heart was beating at almost 220 beats/min. At first when it started I just thought it was kind of strange, and since once or twice in my life I'd felt my heart flutter that way, I thought nothing of it, but after 5 then 10 minutes I was really getting concerned. At 15 I started calling my husband and at 20 minutes of no change I headed out to the hospital in tears and scared sh****ess for my baby. It continued to beat that way for nearly 2 hours. They had to inject me with adenosine to get it to come back down. They were monitoring the baby's heartbeat as well as mine. They tried having me burp, blow up a balloon, blow into a syringe thing, etc before going to the injection, which was their last resort. Both our heartbeats dropped immediately to half, mine remained normal and the baby's was also back to normal within minutes. I received EKG and Echocardiogram and did follow-up with a cardiologist, but nothing ever happened again. They just chalked it up to "gestational fluctuations". My case was extreme and it doesn't sound like yours is as bad, but I'm sure the not knowing is just as scary. Please talk to your doctor and get checked. Good luck to you.
-I.