Back of knee hurts when I straighten my leg or stretch?

So I've had drumline and full band rehearsals basically non-stop for the past maybe five weeks.I think we started marching about three weeks ago. I think I've always had this problem, because the EXACT same thing happened to me in athletics this year. (My first year, this is my first year of marching too.) My drum captian's CONSTANTLY telling me to get my feet up higher..and I get them up as high as I can go. They tell me it should hurt my calves...but it hurts the back of my knees! We do stretches and he tells me to straighten my leg..I've never been able to completely straighten my leg. When I straighten it as far as it can go, my calves and thighs and stuff feel fine..it's just this horrible pain in the back of my knee! he'll pull my foot up when we're stretching, (that's part of the stretch) and it's almost unbearable cause the back of my knee and my ankle hurt so bad...Does anyone else have this problem or advice? This has been long-term, like I said I had this problem since the beginning of school last year...