That's actually.. a large percentage! Whoa..

If you read this http://forum.thefunmouse.com/viewtopic.php?f=1&t=13607, you can see that several people suggested that this is actually normal for this particular mouse when in heat. I actually am going to take that advice for now - if I brought her to a vet when she had an undetectable infection, that would be completely pointless. Will monitor and note changes!

And correct - she has never been impregnated before.

I have never bred before, so I would have no idea what is normal or not- and I am not in the habit of looking at that area for what is normal (although I should) so I would go with what the people with the most experience breeding say. If they say she is in heat- I would do what you are doing and just keep an eye on her. I would keeping looking for a vet though, Just in case!