painful penetration with my fwb

My fwb (23f) tried using a strapon on me (18f) last night. I've never been penetrated by anything other than a couple fingers, so even her smallest strap on was alot bigger then anything I've ever had before. Everytime she tried to insert it it really hurt and It felt to tight to even fit inside me. We had alot of foreplay, and used alot of lube so I'm not sure why it felt that way. We kept trying every 45 minutes or so because I really wanted to do, and we got the very tip of it to fit but it was pretty painful and she said we had to stop since she didn't want to hurt me.
Did we do something wrong? My fwb has a decent ammount of experience and she never tried it with a girl that had as much trouble as me? Could there be something wrong with me? Like a medical issue?