You will never really know for sure what happened last year.
As a comparison, this year our ewes have lambed on average a month apart. Only one of them noticeably bagged up, and another one was actually eating her breakfast when the lamb started to appear, taking everyone by surprise.
Your girl may well have taken during a later cycle, as some of ours appear to have done.
If you can remember when you last saw the ram tupping, and count forwards (or if he has gone, count the gestation period backwards to get a last possible due date), you might get some idea of if she is in lamb.
Here's a handy Gestation Table for you (there's a lambing date calculator at the bottom of the page too):
http://www.tvsp.org/gestation.html If she was mine and didn't lamb this year, I'd give her one more chance next year, then declare her barren if no lambs.