Why does my 5 1/2 month old cry almost everytime a male looks at her? (except her father)?

My daughter SCREAMS when she sees a male looking or talking to her. With her Dad she doesn't (luckily!!) and sometimes she will with one persons another time she won't. The one exception: my father. I know it completely breaks his heart. He was teary eyed today when she screamed so bad she almost stopped breathing. A few weeks ago we were playing, she was laughing at me, my dad stood next to me, she looked at him, puckered up, started screaming and turned purple she was screaming so hard. Someone please help! I'm dying. I know he loves her and wants to hold her but he can't.