My tooth has mysteriously become crooked again (I have braces)?

I'm supposed to be getting my braces off in a week. However, a couple weeks ago I noticed that one of my bottom teeth isn't straight anymore. The braces had fixed it, but now it's crooked again. It appears to be slightly farther up than the rest of my bottom teeth and also looks like it's moving behind them a little bit. The only reason for it that I can possibly think of is that the gum around the tooth is swollen, but I still don't understand how that could have caused my tooth to become crooked, especially while I still have braces. My question is; could that be the reason, and if not, what could it be? Also, do you think I'll have to have my braces on longer, even though the appointment to get them off has already been scheduled? Help please! :/