J.T.
There are SO many things this could be, from totally nothing, to an issue with you and/or the baby. Start by giving your breasts and your baby's mouth a good exam to rule out thrush or any other type of rash. Check her temp, check your temp. Have you had a period since her birth? Sometimes babies will begin to reject the breast due to hormonal changes(but that would typically be both breasts...), or due to a change(increase) in exercise. Are you drinking enough? If you're dehydrated, and baby is having to work harder, she may just be getting aggravated and deciding it's not worth the trouble. She may have an ear infection on one side that makes in uncomfortable to lay that way, she may be cutting a tooth, etc. But honestly, at 3 months, every 4 hours is not unreasonable - so if hse appears full/satisfied/happy, try not to stress about how often she's eating. I'll tell you, I had to pump and bottle feed my first child, he never could latch on due to several issues, and I had MUCH more milk in the right breast(typically got double the volume off that side every time I pumped), and it let down and flowed much more easily. With my latter two children, thank God they were able to nurse, but both began to reject my left breast around 3 months of age. I think that's the point they were beginning to be aware that it was much easier and more satisfying to nurse off the right side. I pushed it for awhile, and ultimately nursed both children exclusively off the right breast for many months. I do have a small fibroadenoma on the left breast, that may or may not have something to do with why they rejected it, who knows. My OB isn't sure, nor is my breast surgeon who monitors the lump(for the time being, we are leaving it alone, it's been there unchanged for 4 years). If you are more comfortable with it, get checked by your OB and have your little girl checked by her pedi - it certainly sounds like it could be in the normal realm of breastfeeding, or it could be the beginnings of some sort of infection(mastitis?) for you or her. good luck and try to relax!