I also get this and have done for a little while. I think it is my own pulse too.
Physio and Midwife told me not to lie on my back after 20 weeks (I think?) and said it was something to do with a big artery in the back and babys weight can squash it which is why you might feel your pulse very strongly or feel faint when lying on back. I cant lie on my back now because I get tingly in my arms and legs.