It can be impossible to tell from belly/milk line until right before farrowing. We had a sow farrow last month with absolutely no signs beforehand. That being said, have you noticed if she has gone into heat since AI? It should be the same time every month so if you did AI when she was in heat on 12/1 she should have gone into heat again around Christmas and again last week (ish). Keep a close eye beginning of March. Milk line may drop a few days to hours before farrowing and when she starts nesting it will be within 24 hours. Good luck!