A stranger pinched my butt

So I was at Wal-Mart in line to check out and a group of guys about my age (in their 20s) were behind me. All of a sudden my ass was pinched and pretty hard. When I looked at them they were all looking to the floor laughing hysterically so idk which one did it. I was so embarrassed and angry and just kept my cool until I got to my car. I called my husband and he is livid and wanted me to go back in and tell the manager but they had already left. What would you have done in this situation? I also just found out i'm pregnant so my emotions are all over the place with little things let alone this.