Husband thinks I'm cheating after seeing me naked. What should I do?

This is a little graphic so I'll apologize beforehand. A couple of days ago, I was really constipated and had a hard time in the bathroom (yea I know gross). I rarely get constipated but when I do, its pretty bad and usually get a reddening and somewhat stinging sensation around the rim of my . But I also happen to get the same exact thing after having with my husband. The same day that I was constipated a couple of days ago, I decided to take a shower and my husband joined as he usually does to save money during times that we both need to take a shower at the same time. He does his thing at one end of the shower while I do my thing at the other end. But that day, when I decided to spread my legs and cheeks to wash my private region and wash my reddened and stinging area, my husband turns and sees the affected area (don't ask how he noticed, I have no clue either). He immediately connected the fact that my was reddened to the idea that I had cheated on him because everytime my gets like that is after we have . I tried explaining to him why it was really like that but he obviously didn't believe me because I had never brought up the fact that constipation does that to me sometimes because its disgusting and not something I would want to tell my husband. He now believes that I had cheated on him with another man and I can't seem to contradict his belief. He believes that what he saw is clear-cut proof. He's overreacting but we're obviously not getting a divorce over this because even he knows hes not fully convinced. But I was wondering how I could shut him up once and for all. Does anyone have any ideas?