It sounds like blister disease but that's, as far as I'm aware having not seen it personally, usually from damp, unsanitary conditions. If her enclosure is kept clean and not overly wet all the time I'm not sure what could be causing it. You might want to Google "snake blister disease" to do some further research on it.

What did you use to treat the mites on this snake? If she's been in good QT since you got her and with mites having about a 30 day life cycle, it sounds like she came to you having been exposed to mites already. Did you speak with the breeder about this mite issue yet?