Sometimes if a woman is multi orgasmic, the muscles around the vagina/in the pelvic area stay in at least a partially contracted state. Sometimes at least one orgasm before penetration is helpful. Just go with the flow and enjoy.
I have had some partners/lovers that had to have at least one orgasm before penetration could occur and some that if it had been more than a few days since the last intercourse that was necessary. It's not that rare, it's pretty enjoyable for all concerned. I wouldn't lose a lot of sleep over it. There's nothing wrong with you. You are just you.