Any updates on this? Hopefully the vet was able to help out and also let you know what kind of milk snake it is as their care varies a bit between the different subspecies. If not try calling whoever supplied you the snake, they should know. Here's my bit of advice in case this problem is ongoing-
Try replacing the heat bulb with a heat pad, as snakes absorb heat most on their bellies (I've seen heat lamps work for snakes, but heat pads are supposed to be more efficient.) Also make sure she has a good temperature gradient- check that her warm and cool sides are in the desired range. She might be in shed, my milk snake tends to go off feed for 2-3 weeks before shedding.
Lastly, maybe she's just a shy eater. If you're only trying pinkies, these can't hurt the snake at all, so try just leaving a live pinkie inside the cage and walking away for a while to let her eat in her own time.
As others have said, having her seen by an exotic vet to check for parasites and diseases is an essential step of figuring this out, but if the vet isn't able to fix things give this stuff a shot too. I really hope she's alright.