I think my fiance is cheating on my and I don't know what to do?

my fiance and i have been together for 11 years now. two nights ago we have some fun in the bedroom together and he tried something new and it felt great. however we never did this new thing together before, it was our first time. he did it extremely well and now im confused on how someone can be so good at something and it was the first time we did it together. i asked last night if he was cheating on me and he said no, but i don’t believe him. I don't know what to do or think.
I think my fiance is cheating on my and I don't know what to do?
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