Toothache a year after large filling?

About a year ago I had insurance for the first time in awhile, got the majority of cavity's done that I needed and then, of course, couldn't afford to keep it. One of the larger fillings on the bottom has been having a pretty bad ache for days now. Any suggestions to what could be the problem? And please don't bother replying if your going to tell me I'm dumb and to go to a dentist, if you don't have the money you just don't, and that's life, so I don't need to hear I need to do something that would be my #1 choice.