What happened was most likely that whoever you got her from soaked her and killed the adults, then you brought her home and in the 3 or 4 weeks you had her, the eggs hatched and then the babies got big enough to see. Unless you have another reptile in your house with mites or have been handling other reptiles with mites and bringing them home, this is what happened. You need to treat the infestation thoroughly to get rid of them, and most people will suggest using Black Knight or Provent-a-mite to kill the eggs as well as hatchlings and adults.