Was it her first heat? As I mentioned in the hub, if it's the first heat, they can go into a split heat, where she won't ovulate the first time, so she'll go into heat again about 2-6 weeks later where she will ovulate. She'll show all the signs of being into heat in both occasions with a discharge and swollen vulva.
I strongly urge spaying, as it will reduce odds of mammary cancer. If done before first heat, you'll reduce the odds to near 0% chance, after first heat the odds are increased, and after second heat, the odds are as though you never spayed.