He thinks I cheated…

Yesterday I was wearing shorts and my bf noticed I good sized bruise on the inside of my thigh more near my vagina- I told him I didn’t even know it was there or how I got it but he really was worried I was cheating on him. I told him it was a bruise not a hickie, and reminded him that in my job as a nurse assistant it’s very physically trialing and I come home with bruises all the time. It just sucked because I felt like no matter what I said he would still be worried, but I didn’t do anything wrong and I’m telling the truth I have no idea where it came from, he knew it was there before I did! I just want him to trust me, and want some advise on what to say to make him feel more comfortable.