Please listen to SLadylani. It could be a simple case of mastitis, but less likely since you're not actively breastfeeding. On the other hand it could be something far worse. If the skin looks dimpled - kind of like an orange peel - you need to see a doctor quickly. There is a form of breast cancer that grows in sheets rather than lumpy tumors and it's often misdiagnosed as mastitis. I don't say this to scare the liver out of you, but to impress on you how important it can be to waste no time seeing your doctor.
Oh goodness, it seems like you're in a lot of pain. With what you're describing I really think it best you get into a doctor as soon as you can. What you have could be a variety of things, all of which you need a physcial exam and possible tests/scans to determine what exactly is going on. Being that you haven't breastfed recently, I don't think it's reletated to that. Are you due for your cycle anytime soon? Do you take the birth control pill? Have you noticed any breast changes such as skin dimpling or puckering, redness? Try feeling for lumps again, it's best when lying down with one arm raised above the head. Remember, breast tissue goes all the way into the armpits. But I still think your best bet is to get checked out by a doctor, it's better that you find out what;s going on now then wait. This websites is here if you have any questions or need support, and it's anonymous if you want it to be :) so there's no need to worry about being personal. Good luck and keep us posted!
Sara RN