Swollen, Red Cyst On Labia. Having Fever, Mucus Draining From Nose. Diagnosed As Roseola. What It Might Be?

My 12-month-old baby girl has a large, swollen, red cyst on her labia . There seems to be a head on it that is dark. She is teething , getting in 8 teeth at once. She has had a low grade fever for a week with clear mucus draining from her nose. I took her to the pediatrician on Wed after she developed a slight red, splotchy, body rash, which was later diagnosed as roseola . I discovered this cyst last night and have treated it with diaper cream, which causes my daughter to cry in pain when I apply it. Any ideas on what it might be?