I had the same thing happen at 27 weeks - no explaination for it, either. Mine was accompanied by lots of BH, though I've been having those anyway. My MW checked me and all was fine, she even did a fetal fibronectin test - came back negative, so it was really just some unexplained bleeding. With my first pregnancy, I had a polyp on my cervix that caused bright red bleeding - I had it removed and that cause of bleeding cleared right up.
How are you doing today?