Is it cheating

so on the 24th I went to this drama thing with my partner and they weren't talking to me at the time and wouldn't let me hold their hand like they normally would they've been distancing themselves from me for a little bit now and I knew I needed to talk to them about it the only way out of the costume room is to go down the ramp without going through the set shop next door so I walked out and saw my partner sitting in a chair with a girl on their lap a ran away and then my partner had her ask why I seemed mad an my partner and I just couldn't and then they went in the bathroom for over 20 mins so I wrote a letter explaining things and telling them if they want to still be with me they couldn't be friends with that girl I don't know if it's cheating can you guys help me with this one I have no clue