Why does inserting my tampon hurt?

I just got my first period since giving birth 6 months ago. I had a csection. I've had sex since then with no pain. Masturbation with no pain. I've even done my own vaginal canal exam and nothing is out of the ordinary. But when I insert my tampon, it hurts! It feels like something is already in there and im just pushing it further up inside me. No, I didn't forget a tampon. No, my cervix isn't super low. And by the time I go to take it out it's almost out of my vagina, like I wasn't able to put it in all the way. What could be wrong?