Me and my ex

Me and my boyfriend broke up about 3 months ago and we were together for 2 years , but recently we got back together. We were broken up for 2 months, and recently within them 2 months he slept with 3 other girls, and the first one was only 2 weeks after we broke up. And today I found out exactly who them girls were. And it just makes me sick to the stomach and just honestly so jealous. Am I wrong to feel like this? He said himself it was nothing. But I honestly cannot Get over it yet as I wouldn’t have done nothing of the sort so early on into the breakup. He said it meant nothing, he only did it because he was heartbroken. But it’s on my mind 24/7 and I just don’t know what to do or think. Should I even be bothered? I understand that he wasn’t with me at the time, but I just don’t know how to cope with this feeling