As stated, it's just a matter of getting accustomed to his length, girth or shape...one or all of these issues. The good news? It can happen in one night if he knows what he's doing. Regardless, you'll have what I always call deep fucking soreness. It's sore inside, especially around the cervix but it's a GREAT sore. You definitely know you've been fucked by a big cock. Often, I feel it for a day or two and the soreness makes me remember the things he did to me to make me sore like that and it makes my pussy tingle!
As for the shape, few men have a straight cock; they tend to curve one way or the other, so it may be the head of the cock hits a certain place in your pussy repeatedly. Just like when/if someone pokes you over and over in the same spot, the poking gets tiresome after a bit, so just change position. Other than that, know that your pussy can, and will, adjust/adapt/conform to any shape necessary and, if you're lucky, enjoy feeling his cockhead pushing open your cervix! God, what a feeling!