overly jealous

My boyfriend's best friends (who happen to be girls) and him went out and one of them posted a picture of them sticking their tongues out and touching them to each other's. I saw the picture and was a little upset (??) but it was a whatever type thing. The same girl to share the photo also put a picture of her sitting on his lap on his story. I knew it was her because he never posts stuff. I also wasn't completely bothered by this. Then, an hour later, someone sent me the photo with their tongues out and captioned it "yikes.." and I lost it. I got so upset. I sent it back to my boyfriend saying "when people start sending you this shit 😕🙃" and I couldn't even talk to him. He asked if I cared, if I wanted him to come over to talk, and I straight up told him I didn't want to see him or talk to him at the moment. I told him I was upset. Why? I'm (pretty sure) he would never cheat on me and I don't consider what he did cheating unless he full on made out with them.. idk. I was just so angry and anxious and I couldn't fall asleep for hours. What do I do? How do I calm myself down for the future?