He yelled at me in front of his family

So we were shopping with one of his  brothers, sister in law and mom. Well his brother asked me to go find a Christmas present for my SO and I told him I was going to look for a phone case and he said okay and then he walked around and found me looking at the movies for him and he yelled at me and said I was sneaking around and asked if I was cheating and hiding something. I said no you don't need to worry about that you know it. Well he later yelled at me as were walking back to his family and said I am doing something wrong and don't let me away again. I've never cheated on him or hid anything. He has all my social media passwords and he knows that I never would. Period. What do I do? What do I say to him?