MamaJenn -- it could be that your baby is done eating after those short 5-8 minutes. Some babies are highly efficient and get all that they need in 5 minutes or less, even! Some babies are all business too and all they want is food from the breast (while other's are more comfort nursers). This is why we always suggest that you watch the baby, not the clock.
If your baby is gaining 1 ounce a day then she's getting enough milk from you (between 0-4 months babies typically gain about 4-8 ounces per week) & there is nothing wrong with your supply. You may just want to end the nursing session once she starts pulling off and see how things go from there. Or switch her to the other breast. Some babies get frustrated when the fast flow of milk slows down. If this is the issue then let her nurse the 2nd side and then try to put her back to side #1 after she's done with the 2nd side.
It sounds like you're doing just fine!