Why is my dog acting like this?

My family and I have a two (almost three) year old Rottweiler. She's usually fun, oveable, and acts like she's a cat. But a few days ago she changed a bit. I went over to her and she started to growl at me. I slowly backed away and even when I was out of the room, she continued to growl. She did it anytime anyone came around her, but her nub (tail) was wagging, so my dad didn't think anything was wrong. My mom thinks she might be going into heat. Anyone else have any ideas? I'm scared to pet my own dog. :/