Would this be considered cheating?

So, me and my partner of exactly 4 months now went to a Halloween party today. During the party at some point, us and a few other friends were hanging out in the host's bedroom when my partner climbed onto the bed and lied down. Then, another girl who I've known to have a thing for my partner also lies down beside my partner and starts to stroke his face right in front of me. My partner sorta laughs and says, "What are u doing??" but makes no movement to move away. At this moment, I couldnt stand it anymore and went away, leaving my friends and partner. Not too long after, my friends come out saying, "ugh. i had to leave. they were literally climbing on top of each other". They stayed in that room for almost 30 mins, being rly loud... When they finally came out, I saw my partner stroke her face softly with his fist and smile, with that same expression that he gives me (the loving one). My question is, am I over reacting? I didnt actually see them climbing on top of each other, but I feel like he's got feelings for her too. What should I do? :/ Please help...