Pulsing/vibrating below breastbone?

Britt
I'll be 37w tomorrow and the past few days I've felt a rapid pulsing or vibration on my right side, at my bra line near my rib cage. It's happened a few times in past weeks but has definitely gotten more frequent.
It's not hiccups (too fast and high up) and is usually when I'm laying down or taking a bath. I can usually get it to stop by getting up and walking around. 
Anyone else experience this and find out what causes it?