hi im am 20 years old and for the past 2 weeks every time i bend my leg and then straigten it it cracks and mildly hurts. My knees have sometims cracked before but never this consistant and it is only my left leg that is doing it. It also cracks alot when i sleep i sleep with my legs bent and straigten them out throuout the night and it wakes me up because it hurts to straigten one leg. what should i do?