A pulled muscle
There’s a lot of thrusting, a lot of angles, a lot of bouncing and a lot of things that can lead to a pulled muscle during sex. For men, in particular, it’s easy to pull a muscle in their thighs if they’re on their knees thrusting away.
What to do
Luckily, most pulled muscles due to sex can be treated the same way any pulled muscle is treated: with some ice packs, elevation of the pulled muscle, and rest from whatever activity caused it.