painful sex ????

I finally made love with someone other than myself(new partner and well endowed) for the first time in about three months and at some points in missionary it actually hurt too much the deeper he went in and so we had to pause. But this morning when I was on top, as well as when I was on my stomach last night, it felt really fine with no pain. I'm confused and his isn't the only time this has happened. Anyone have  Should be worried?!...