Boyfriend thinks i'm cheating because of sunburn. Need answers

F24, no known medical conditions linked to this.

I spent a lot of time outside yesterday in a bikini top. I got sunburned pretty bad all over my chest, including some of my breasts. For some reason, the sunburn on my breast bruised and it looks like a hickey. It's the ONLY place the sunburn turned into a bruise. I don't even know what to tell him to defend myself at this point. What caused my sunburn to bruise?? And why only there?