Me (27M) and my gf (21F) have been dating for close to 6 months. I went to see the results portion of my gfs Hackathon over the weekend. Her group had worked for 16 hours straight the day prior to create an app. As we were watching the presentations of other groups, one of her group members puts his arms around the chairs of two of his group members on either side of him, one of these chairs was my gfs. After a few minutes he took his arm off the chair of one group member, so now his arm was just around my gfs chair. It bothered me a little that this was happening right next to me but I decided not to say anything in the moment. After the contest, as we were leaving I asked my gf what that was all about and she said that she didnt even notice it was happening (which I find hard to believe since we were watching presentations in the room for over an hour). Additionally, she mentioned that they were talking to each other for 2 or 3 weeks about 8 months ago but nothing ever came of it. She says they only ended up kissing and that was it. This added detail made the situation even worse in my eyes that she allowed this to happen, right next to me, with a guy she had feelings for it the past. My gf sees this as no big deal and thinks I am making something over nothing. Am I overreacting over this whole situation? Is putting your arm around someones chair harmless?
Edit: Thank you guys for all your input, large range of responses here but it was very easy to tell that I was indeed overreacting. We got together and I apologized to her for even making a big deal of it. I have also sought out psychiatric help as many of you pointed out that I should. Thank you all for the firm, yet helpful reality check. I will grow very much from this.