Just going on what i have read the doctors say on this website. It is more likely that it is an HPV strain that has been dormant in your body for many years.
Yes, once you are infected with a strain, you cannot be reinfected, BUT the original infected can become active again. You were having good paps after finding out you had high risk HPV, meaning that the virus was laying dormant in your system. You never get rid of HPV, but after a while your body fights back and you are then deemed clear - so you are no longer contagious and the virus is no longer detectable, but sometimes the virus can become active again like in times of ill health (such as a minor cold) stress or bad luck.
If it is a new strain, it could be one you've already had before that has only just become active in your system. Sometimes HPV can lay dormant for years after exposure and randomly pop up. This, however, is relatively rare. It doesn't mean he cheated. Ask your Dr, they will do tests to find out the strain