Firstly re check your dates, was she served just once on that date or more than once until she went off? We use natural service and AI, on the natural the sows are mated twice a day for two days.120 days is really the max they can go, you may get one going over that but it is rare. If she starts showing signs of distress ie: trying to farrow/pushing/ pain etc for a long period and nothing happening, unless your expierenced with pigs call you vet, he should examine her and may well give her a shot of Oxytocin to get her going. You will get lots of advice on here some good, some rubbish. She's your sow so you should know her farrowing pattern, if it is following the same pattern as last time all should be well, 99% of the time sows follow the same pattern when they farrow, if they dont, then normally there is something wrong. When she starts farrowing you really dont want a long protracted job, if you are waiting for more than 3/4 hour to a hour for the next piglet to come then chances are there is a problem, two coming at once, still born, mumified backwards etc, thats where Oxytocin is very good as it will get the sow going, however do not give it until you are sure there is nothing stopping the piglets coming out. If in doubt get your vet out..
Hope all goes well. We farrow hundreds of sows every year and I am still learning!! Good luck.....
Ps. Forgot to say a sow/gilts temperature drops a half to a full degree before they are due to farrow, thats always a good indication of whats hapening.