Is my husband cheating?

My husband has a high sex drive. Since having a baby we haven’t had as much sex but when he’s in the mood nothing gets in his way! Last night, he started fooling around with me. I had taken sleeping pills and told him I was really tired but I got in the mood eventually. When he asked me to get on top I almost slipped and fell while changing positions but quickly recovered. He told me “you are tired. Go to bed” and in an instant he no longer wanted to sex and the mood changed. This was unusual. This never happens I asked him about it and first he said he thought I said I was sleepy and tired which I never said. When I kept pressing for an answer because I felt insecure he said the truth is because he has been in pain lately especially his knees. Me slipping somehow reminded him of this and he was no longer in the mood. I don’t know. That’s such a weird answer. I let it go and focused on having a convo about his pain but I read that a sign of cheating is when your partner no longer wants sex with you. No other cheating signs other than him picking a fight with me this morning over something super dumb. His schedule is still the same.