what can i do for a swollen leg on a minor

My daughter has a painful swollen leg. I took her to the hospital and the took xrays of her knee, ankle and her leg, they also did some blood work and everything came out fine. Im very worry. I dont know what to do or what can it be. She doesn’t have a bruise n she didn’t fall. Can you please help me?
Thank you in advance
Marilyn