I lost my virginity 3 months ago with my boyfriend and we've been having sex regularly since then.

But each time during the first penetration it would feel uncomfortable or sometimes painful like a sharp feeling. But then after a while like 30 second ish the pain would go away and it wouldn't feel painful at all.

After that even if he reenters when we change positions it won't be painful.

It's always the very first every time.

Why is this? Am I not aroused enough at the beginning?