Why is my girlfriend mad at me for being honest?

Hi people, can someone explain to me why my girlfriend is so mad at me for telling the truth to her. Today she was trying on jeans and asked me if they made her look fat, so I told her yes. She got really mad and started to cry, I told her to stop crying because she looks really ugly when she cries. Then she hit me and I told her that her flimsy arms won't do much damage to me. She started to cry again, so I pulled out here pepper spray, shouted "Sally! Sally! Why are you doing this to me?" and then I pepper sprayed myself in the eyes. She ran out of our apartment screaming, what is wrong with her? What should I do?