Internet speed slower than usual, have i hit my download limit?

Internet speed slower than usual, have i hit my download limit?

Normally when downloading on Steam i get around 600-700kbps, which isn't great (due to living in the middle of no where) but is certainly usable and can download hefty file over night. However, since probably last weekend the speeds have dropped loads, even normal webpages take time to load and the download speed jumps all over the place, but is normally around or below 100kbps, however it did very briefly go up to 500kbps the other night for a very split second, then plummeted again.

I put this down to my USB wireless adapter at first, i tried moving it about, re-starting it and all sorts, but it said the signal to the router was strong and nothing changed. So i made sure everything else that could connect to the router was off or not connected, still no difference.
I tried downloading on my old laptop, and using just Firefox it was getting even worse speeds, around 20 - 30kbps.

So now i'm thinking maybe i've hit our download limit (i'm with AOL and have had the account probably > 5 years now). Is this the sort of thing that happens with you hit the limit (this would be the first time)?

If so, when will it go back to normal, the beginning of next month or our next payment date?

Finally, if you don't think that this is the problem, what do you think it could be?