I'm thinking its possible she had a small amount of the lice when I got her, and then a combination of the darker, warmer environment of the stall encouraged them to thrive, plus she went from the pasture, high in vitamin A, to grass hay. I just got her on the first of February and she hadn't been de wormed by the previous owners for almost a year.
I didn't get a chance to call the vet and see if we should disinfect the stall, but my barn owner has spray and a sprayer and offered to disinfect her stall for me. So we'll probably just do that anyway.
I have been applying MTG daily, and that should take care of any rain rot. She was very, very itchy though, which I thought was inconsistent with rain rot?