I'm 10 weeks and I started feeling like a vibration in my stomach, but more so my upper stomach. No idea if it has anything to do with being pregnant, especially since it's not lower. It's kinda like if I had a phone in my stomach on vibrate and it happens randomly during all parts of the day.
I haven't had this happen in my pregnancy at all yet but I did have it happen to me before I was pregnant (about a year ago). It felt like i was sitting on a phone or something and it came and went for a couple days and I haven't had it since. Everything that I have found out about it suggests that it's either an increase in blood flow to the area or possible small muscle twitches/nerve activity. My guess would be it's from pressure on a nerve or just increases sensitivity down there that's allowing you to notice all the fluids flowing and moving.