First thing to check for before jumping to anything major is to check if there is anything your horse can get hurt on in the pasture? If there is, try to get that removed or fixed.
If there is nothing for your horse to get hurt on, then its probably the other horses. Then watch how the other horses are with your horse. If it seems like the other horses are beating her up badly then see if you can move her to a new pasture at the same place.
Some things may seem like a big deal but it may be nothing. So just check for those things first before moving her to a new barn.