Try this converter:
http://www.neildewhurst.com/hand-converter/
..or if you already are, with some better settings.
He's certainly representing top or second set, but he could also be semi-bluffing. There's a small chance he's trying to push you off a biggish hand or even thinking he is value-betting with top two, as bet-3-betting the flop would scare almost anything as that would look like a set. You do have to take your image into account here.
My reason for folding would be that the 1800 bet totally commits you, and he knows it.