A guy called me a slut! HELP! what do I do?

I was at the pool today with one of my closest friends. I was wearing a bikini. I guess my boobs were showing or my @$$? I was in the pool goofing off with my friend. Some random kid (maybe 13-14) he yelled out and pointed to me, "SLUT! YOU ARE A SLUT!" I was pretty shocked he meant me? I WAS CONFUSED and I yelled at him, "what?? are you talking about?" He giggled with his friends and gave me a evil smirk. He was like looking at me and kinda staring :( Maybe my breasts were showing but I don't think so. It hurt me.. I got upset.. What do I do now? And why did he say that mean stuff? and I didn't know him @ all! I am 13 :) if that matters