Your dog is in heat. During this period of her estrus cycle, she will emit a red discharge, similar to when a woman has her period. Shortly after, the bleeding will stop and the vulva will remain swollen. This is usually when the female will accept a male. I strongly encourage you to keep her indoors unless on a leash, and far away from male dogs. Even if your yard is fenced in, don't let her out alone. Dogs are crafty when it comes to breeding. When her cycle is oover (in about ten days) I strongly encourage you to have her spayed. There are many health benedits to spaying a femmale dog, such as reduced risks of certain cancers. It is also beneficial becaue you aren't contributing to the hundreds of dogs placed in the shelters.