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.
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!