Ok first off welcome to the forums!
I'm probably about to lay a lot of questions on you, so get ready!
First, it is certainly possible she's pregnant. But it is possible she isn't, too. In the "Think Before You Breed" section you can find pictures of people with pregnant rats and you can compare to your own. Generally, the way to tell is they get a swollen belly and something happens with their nipples. I think. Ok honestly I'm no expert on this so I'll let someone else field further questions.
What kind of noises is she making? Does it have to do with her breathing? If so, it may well be an upper respiratory infection (URI) which can be cleared up with a vet visit to get some Baytril.
From what I've read, generally if you can see the bugs then they are lice, not mites. However the treatment is the same either way. Most people will recommend getting Revolution online. I can scrounge up some websites for you if you need them. There are a couple that don't require a prescription. Flea spray (Or any kind of spray) is not recommended because the spray can be damaging to their sensitive respiratory systems.
Finally, have you made sure to quarantine your rat? With a rat likes this (And she sounds like she was meant to be a feeder rat), she could have all kinds of different diseases and quarantine is very important to make sure they don't spread to your current rats. The mites do imply that she may be a little sick. If you can, try to keep her in a separate house for 3 weeks. If that's not possible, the opposite side of your own house and be very careful about handling one rat and then handling another.