COngrats on your pregnancy! The best part about all of this is that if you did deliver, your baby would survive. I had a previa when I was pregnant and was ordered to bedrest. Stay off your feet...I noticed when I was moving around a lot, I would have more contractions. They are becoming more frequent now, because your body is prepaing to deliver! Best of luck to you!
I don't have placenta previa, but I've been getting braxton hicks contractions since 20 weeks or so (I'm 30 wks. now) I get quite a few every day. As long as you know they are BH and not the real thing, I wouldn't worry. My midwife said BH don't do anything except tone the uterine muscles.