Amps, don't panic. Bleeding is way more common then anyone mentions.
I had almost constant bleeding from 13 weeks to 24 weeks (including once when I lost 2.5 litres in a 3 hour period) and ended up having 3 blood transfusions and an iron infusions. I am sitting here waiting for my little miracle to wake up for a feed. They never quite worked out what the problem was, but as Bella mentioned it was either a subchorionic hematoma or a retroplacental hemtoma.