Not sure if this is any consolation but a friend bled a bit throughout her pregnancy if she over-exerted herself. Sounds like a perfect excuse to put your feet up and get someone else to run around after you!
It's still worth speaking to the midwife though or asking for another scan to put your mind at rest.
According to my GP it's something seen commonly in women on the pill or pregnant. The cells on the 'inside' of the cervix are of a less sturdy type than those on the outside, generally. During pregnancy, the less-sturdy cell type covers a greater area, and is often found around the 'hole' as well as inside it. Any increase in blood pressure can cause the skin to split a little and you get bleeding.
Most reassuring, especially when she drew me a picture!
For those of a technical bent, the sturdy cells are called squamous epithelium (like inside your mouth) and the less-sturdy ones are columnar epithelium (like inside your gut).
You do still need to get this checked out pronto but I have cervical erosion and I bled on and off between week 20 and 25 and this sounds similar. Mine didn't seem to have any obvious triggers but it took them a while to check me for erosions and I worried a lot before I know.
Do yourself a favour and ask them to check for it - it will send your mind and rest.