Is he Cheating?

So I can sometimes Overthink things but  it would really help if I could get some help and support.. 
  So a couple weeks ago I found out that my husband didn't want to kiss me. Because he had some mysterious rash on his lip and he didn't know what it was .. We ended up getting him cold sores ointment for the outside of his lip and it's started to go away so we continued to kiss as usual and now I have the same itchy bumps on my lip that won't go away.. So I'm wondering if He's Cheating on me and getting this from some other girl because  I know I don't have any STDs!!! Or weird rashes so how else could this happen and what it it?