Hi, A Few Weeks Ago I Went Into My Left

Hi, a few weeks ago I went into my left splits, with a good warming up before. I also stretched. But since that moment I have this pain right underneath my butt. If I flex this muscle it hurts from there to the back of my knee. It's at the back of my leg.

I have got this for 5 weeks. I have dance 3 times a week, so I try to stretch it a little bit but not too much. My question is: how long will it takes to heal and what can I do to speed up the process?

(Sorry for my English, I'm German.)