I don't think there is any way that you could have the same clogged milk duct for weeks without it becoming mastitis. So either you have had multiple bouts (I had them every few days when my son was two to six months), it is something else, or if you have the signs of mastitis, you probably need to go in. It can become really dangerous if you do have mastitis, I know of one mom who was in the hospital for a week because of what it became.
I would not go to the ER, just go to a doctor's clinic, it will probably be around $100, plus maybe $30 or 40 for an antibiotic. The ER will be a lot more, and you can still be seen within an hour or less at a clinic usually. We don't have insurance either, and I have done this for other things. I also once got an antibiotic prescription from our pediatrician for mastitis, although I got rid of it without taking it (used some natural stuff), and it had already started to get better before I got the antibiotics. It lasted a few days and then the fever, pain, redness, and hot sweats/cold chills went away.
Also, if your clogged ducts are recurring, it could be that something is setting them off. It might just be the fact that your child isn't getting it all out, since you are only night nursing, or it could be something in your diet. Dairy was what got me, as soon as I cut down on dairy I had no more issues, and I have never had dairy problems before or after that.