So, I may be worrying about nothing, BUT... id like to hear what other people think.

So me and my boyfriend of 1 year just had sex a week ago (and due to my time of the month, it was the only opportunity we had so far). We are exclusive, and im on the pill, and neither of us never had any STDs, so we didnt use a condom.
Before me, he has never had unprotected sex. but he has other health problems not related to sex, so he frequently sees a doctor, has blood tests, etc., so i would have assumed they would have caught something, if something was seriously wrong. (maybe that is ignorant thinking?)
And I had only one previous partner, which was in a committed relationship, and the only time it was unprotected was when i was on the pill, but we were both virgins. (i later had to stop the pill and we always used condoms).

Anyway, so heres my question:

3 days after me and my current boyfriend had sex, he experienced what he explained as a redness or chafing. He said it didn't burn to urinate, and didnt really itch. Just kind of red. but I was very surprised, because Im completely fine. Ive never had any stds. and i have no reason to believe i could have any.
He mentioned the possibility it could be his exzema acting up, especially because the drastic season change, its been warmer, more opportunity to sweat, etc.. and this weekend (when he first experienced the chafing) he was doing some physical activity and could have just sweat a lot..
He also told me this has happened once before when his exzema first started, that he had this rash thing near his genitals, but it only lasted a day. This time, it has lasted 4 days so he is concerned, its just starting to go away now. (granted, its been really warm these last few days).
Now he is skeptical about MY health, although Im confident its not from me.
Does this sound like an STD, or anything to worry about?