It could be your meniscus, but if this is what you'd done I'd have thought that you'd have felt it happen. I had mine repaired this spring. When I did mine I knew straight away that something was wrong and the pain went from sore to unable to put weight on it in about half an hour. On the other hand when the surgeon went in he said that some of meniscus and cartilage damage wasn't recent (don't know when I did it though).
Another possibility is a seized up IT band. I went to Whistler this summer and after about a week I started having knee problems. My knee would buckle under me when walking, sometimes just seize up for a split second (with sharp pain) and swell up every day. I thought I'd f**ked it again as I should have taken more time off between the surgery on my meniscus and going skiing again.
Went to the physio who started loosening up my IT band and the problems almost gone away. It still swells after exercise (particularly bad the following day) but doesn't hurt while I'm actually doing it. After a bit of IT band stretching it starts to go away again (get yourself your own foam roller if this is the problem, its worth it). I'd never had problems with my IT band before surgery and had no idea that it could cause such problems. On the plus side it is pretty easy to rehab.
Never done my ACL so don't really know the signs and symptoms for that. I'd expect you'd be having a lot more trouble if it had fully torn though.