BF told me I need to lose weight?

I'm a healthy weight of 55kg (8stone 6 or 121lb) I think I'm Chubby. I normally ask my bf if I'm fat he says no. Today after sex when I went to Lee and came back he told me to turn around to see my butt but I said no and tried to cover myself I told him to not look at my fat and I was trying to cover my belly. Then I started to squeeze it and I was like OMG I'm really fat. And he told me that I should start working out. I know I'm large and wish to be smaller around 45kg which I think is perfect (my opinion) I'm super self conscious and never show my belly and my fat. I think it was mean of him for saying it but I'm happy he's honest. I want to know your opinion.