[Ennui.]

Getting warm or hot can happen during sexual activity because you're exercising and raising your body temperature, so it could be because of that. She might have been moving quickly or pausing to get the right angle or pace to help make things feel more pleasurable for her.

I don't think you hurt her, especially since she said she was perfectly fine. If it makes you feel better you can bring it up to her. Let her know that you know she said she feels fine but you want to make sure you didn't do anything that would make her feel uncomfortable or in pain. Let her know that if she ever does feel pain or discomfort when you're doing something sexual, you want to know so you can stop or do something else. Communication is important in this type of situation for sure, and she may be able to reassure you that everything is okay.