That doesn't mean she was force fed. BP can develop a taste for prey items. If she's taking the mice without being forced, it's likely that she has a taste for them or doesn't realize the rats are for eating.

Another note on the weight - her size also doesn't point immediately to force feeding. In fact, many breeders will maintenance feed the animals they are planning to sell because it's cheaper, customers don't want to pay a higher price for a larger animal, customers will buy a smaller snake over a larger version of the same snake, blah blah, etc. Example - in November, I purchased two BP from a nearby breeder. Both animals have been eating fine for me and are growing quickly with once a week, 10-15% feedings. I recently visited the same breeder and purchased three more snakes from him, all of which were born around the same time my original two babies were. The trio haven't been fed to 'grow', and are 1/3 the size of the two I purchased in November.

Some things you can do to help switch your snake from mice to rats is to get some used mouse bedding (possibly from a local pet store?) and rub it on her rats to transfer the smell. You can also bait and switch - offer a mouse and rat at the same time and swap the 'lead' prey item when the snake gets ready to take it.