The backs of my Legs are aching. Need help please?

I have been having this problem for more than a year. The backs of both of my legs have been aching and hurting. It goes on for a couple days and then it will go away for a while, then it comes back. When I sit on the floor and stretch it hurts even more. It almost feels like it is burning or pinching. It is both legs, down the whole leg. I can feel my hamstrings and all the muscles down the back of my legs pulling. I have been to my general dr and she told me to stretch and didn't act like it was anything. It started when I was pregnant so she thought it had something to do with my sciatic nerve or something. But now my son is a year old and it hasn't gotten better. I have also been to a back dr because someone said it might be related to my back. They did x-rays of my back and legs and examined me and found nothing. So I am not sure what to do. I have tried hot baths, tylenol, stretching, different shoes, etc. My husband thinks it's stress. But I don't see how that can be. Please help if you have heard of this. Thanks!