You dog does some of her talking with her tail. It's an important piece of her body language, helping communicate her moods and feelings. When she tucks her tail under her belly, it's not usually a good sign -- it most likely means she's afraid or upset about something.
If you see your pup's tail tucked in between her legs, the most likely reason is fear. When she's scared of something, she lowers her ears or points them back and hides her tail. The "something" could be a clap of thunder, the approach of another dog or walking into unfamiliar territory, such as a friend's house or the vet's office. Take it slow and easy when you think she's scared; frightened dogs can lash out and bite unexpectedly out of fear.
Your pup might be telling you that she recognizes you as the alpha dog when her tail is tucked. This might occur when you reprimand her for eating your shoe or when she's in a vulnerable position, such as on her back. When you roll her over to scratch her tummy, she might tuck her tail as a sign that you're the boss and she trusts you.
Dogs get stressed in different situations, but a typical sign of stress is hiding the tail. If you're packing your overnight bag and she associates that with being boarded at a kennel, she might tuck her tail and move away from you as you pack. Adding another animal or a new person to your household also can stress your pet, and she might slink away to avoid the situation. Be patient when you coax her out of her funk because stress, like fear, can lead to aggressiveness.
New and temporary situations might lead your pup to tuck her tail until she figures out what's going on. For example, if an unfamiliar dog comes to do his version of saying hello -- by sniffing her privates, of course -- she might feel uncomfortable and tuck her tail to prevent access until she's had a chance to say hello to him. A house full of people for your best friend's baby shower might make enough of a ruckus to make your dog feel uncertain about her role while the party is going on. Even if she approaches people for a pat on the head, she might keep her tail tucked until they leave and the house is hers again.