Aggresive Dog.. Possible?

My dog is about a little over a year. I got her about 4 or 5 months ago from a rescue shelter. She is part Rott part Bulldog. She really is the sweetest thing. The first aggression I had ever seen was probably about a month ago one of my boyfriends friends came over and she kinda snapped. To be honest I didnt think much about it just thought she didnt think to much of him. The last couple of weeks I have been noticing more of this. She will growl and snap at people that she has even met before. This is the thing I find odd, she stays at my boyfriends half of the time and at my place half of the time. She has never snapped at anyone at my house, other than a little barking at a stranger. She has never snapped at my parents or a few of my friends that she has met, but over the weekend we were at my boyfriends and he had a few people over and she was acting up. She was growling at people that she had met before. I was thinking maybe it was a guy thing because she wasnt growling at the girl there. Then the next day my boyfriend said she had snapped at one of our girl friends. I took her the next day to a friends (different friend) and she was fine, she acted normal no growling at anyone not her mom or step father or anything. Also, I take her to doggie day care twice a week to get a little exercise she loves playing with other dogs and she has never showed any agression to any of them. So I am just clueless of why she is doing what she is? Any help would be greatly appreciated.