Hi jades dad,
If she has not been spayed then this may be what is going on or your girl may have vaginitis, (a bacterial infection). If she is licking the area,or scooting, that is another sign. I would bring her in and have them do a swab of the area. Other possibilities may be a urinary tract infection. Either of these would need antibiotics for a few weeks. Take her temperature to see that it is normal. A slight fever may indicate infection. Normal for dogs is 100.5 to 102.5. Also bring in a urine sample to check for a urinary tract infection.