Suka, I had a couple of instances of this in both pregnancies, with penetration, both times in the same position, and from what I could figure out, from MDC and talking to my midwife, is that the bleeding was due to to my cervix being more vascular during pregnancy. Also, when this happened before, my midwife told me that as long as it didn't last long, and there was no cramping & it was not heavy like a period, it should be OK.
Hope this makes you feel a little better, just go with your gut on something like this. If you feel like you need to be checked out, get checked out.
Amanda, livin' and lovin' in Memphis w/ Brandon & Eliza (8.15.10)! 6/2009.