Lately, often when my LTR and I are hooking up, she'll stop and ask me if I'm okay and say I look upset.

Obviously, I'm not upset mid-fuck.

If she was just a fuck buddy I'd probably make a joke like "I was fine till you started talking" or something. But the fact that she has asked several times lately makes me think she's actually concerned about something.

Last night when it happened I just said "you keep asking me that" and went back to hooking up like it never happened.

What's an appropriate response to this if it happens again?

Thanks.