What's With The Evening Dog Barking

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My 18 month old Yorkie is a really good girl. She is in basic training at the moment and learning really fast. Overall, she is well behaved. No major problems at all. I have had her spayed in early October of 2008. (I only just got her in August and had to wait for her to go out of heat) When we walk during the daytime she is fine. When we go for a walk at night, usually in the same area, even the same direction, sometimes not, she barks at everything. Even at nothing, that I can see anyway. It is slightly embarrassing. It is almost like she likes to hear herself bark or something. What is troubling is that often, during these barking sessions, while she is walking, the barking is either started, interrupted or ended with small growls. This indicates aggression. She is not exactly aggressive. She is very playful and loving. When other dogs, people or kids are around, she is wagging her tail and wants to play and give kisses. She doesn't bark when she is inside or during any other times. Unless during the day and she sees another dog in the park when we are playing. This has only just started happening recently, within the last week. Other than the barking, her body language hasn't changed. She is exercised regularly everyday. Does anyone have any suggestions or opinions? I am unsure what to do about this. Is she thinking that she is protecting me perhaps, since it is dark out? We have walked at dark before and she wasn't doing this.