I have a 2lb Chihuahua that is 8 months old. I am used to dogs having a significant amount of discharge when they are in heat but I haven't seen any with her. I also have a male Chihuahua, he is fixed, but he hasn't acted like he was more interested in her or anything of that nature. She hasn't changed her personality or acted like she was in heat, as most animals seem to do. I bought her a diaper to wear and haven't seen anything in it either. I'm at a loss.
It's possible she hasn't come into her first heat yet. It's not uncommon at all for smaller dogs to come into heat between 6 months and a year. She could just be a late bloomer. =)
If you're truly worried about it,take her to a vet and ask. ^^