Soreness after sex 5 1/2 months PP?

Hubby and I had no problems resuming sex after our daughter was born. She was born via c-section and once we resumed sexual activity there was never really any pain, just moderate soreness once or twice, but nothing terrible. I'm now 5 1/2 months PP and we hadn't had sex, probably for at least 3 weeks due to my husband being busy with work, our daughter being sick and miserable from teething, the holiday etc. When we had sex the other night, I was so sore that I literally had to go run a bath and sit in the warm water for 20 minutes! We took it easy and it didn't hurt at all while we were having sex, just after. The only time it ever hurt like that was at one point during my pregnancy when I was very swollen. Should I be concerned? Is this something I should talk to my doctor about?