Heart palpitations

Im currently 13 weeks and randomly started having weird racing heart/heart palpitations while laying down last night. They last 3 to 5 seconds and happen quite often. It doesnt matter if im on my back or side but they dont happen when im sitting or standing (at least not yet). I know it can be caused by the extra blood flow but ive never had it with my other pregnancies so it worries me a little. I have a little caffiene during the day (under the 200mg) and im not anxious. Ive been having a heck of a time trying to sleep at night and im super tired constantly (all which i know is pretty common in pregnancy). I have a history of low iron and wasnt sure if that could be a cause of all of this or not. Has anyone else experienced something similar?