Painful sex...

My husband and I used to be able to have sex no problem. I had a baby by c section 6 months ago and feel that I'm all healed up. But now, sex is painful and we can only get the tip of his penis inside of me. It feels like there's a wall up inside me and he can't go any deeper. I get turned on, and foreplay feels good, but sex just isn't working. What's going on?