When did you bring her to the stallion? Is it possible she could be due late May. Mares each have their own time to foal (within the extremes of 320 to 370-380) so some go very early, others rather late. I believe environmental and nutritional conditions also effect the foaling date.
I agree with LadyDreamer to try and get a date as the not knowing will drive you nuts! This year a maiden mare of mine bagged almost 8 weeks prior to foaling (we weren't certain of the exact breeding date
either) She had me awake watching the foal cam forever! Between the tail switching, laying flat and up and down; a lot of sleepless nights spent on the couch with one eye on the cam and one trying to catch a wink. Of course she had the foal the one morning that at 4 a.m. I decided to crawl into my warm bed! Hubby woke me at 6 to inform me of the surprise in the barn.
Good luck! Happy foaling!