It happens to me sometimes, not pregnancy related. I would actually say that I get some sort of bleeding almost every cycle. I've mentioned it to my ob/gyn, and she wasn't really concerned... just told me to use lubrication. Now that I chart and understand CM more, I'm beginning to understand more when I need the lube, and when I don't. (I try to avoid it because I know it can effect the sperm reaching the "goal!"

) Sometimes even when I feel "wet" down there, apparently it's not wet enough... So when I know I'm not fertile anyway, I will sometimes grab the stuff just in case.
Before I charted, if I bled more after DTD chances were that I was going to start AF in the next couple days. Now that I am charting and know when I will start, I haven't tested this theory yet.... Because really,

in these cases it usually wasn't just a little bleeding when wiping, it was more....

and since the results are not fun (for DH or me!) I've just been avoiding DTD before AF!