Swollen labia after sex? his or my fault?

Neither: Common. And who lays blame in something so enjoyable? She might simply be excited, or if he's rough, she may ask him to lubricate more or be gentler. There's no risk -- keep the communication open and sex will get better and better.

No blame: I agree with previous answer. Instead of laying blame, seek and answer or solution to minimize the swelling part of which is very normal. The lania are composed of vascular tissue, which like the penis, can become normally enlarged or engorged in the course of sexual intimacy or excitement. If this is bothersome and/or mechanically interferes, consider lubricants or estrogen creams.

Both: It might just be you got really excited.