Why are my legs aching?

I’ve been experiencing pain on my legs for about two nights already and I can’t really find an explanation. I am 17, about 5ft tall and I weigh about 110lbs. At first I was wondering if maybe it was the weight of my leg pressed onto the other but then why would they both hurt? Also, it IS that time of the month.. Maybe that has something to do with it. Anyway, the pain seems to be coming from around my knees, this is the first time it’s happened.