why does my leg keep going out?

For a few days my leg has been killing me anytime I use it. I can walk fine but if I try to stand up or do anything where I have to use the muscle on the top of my thigh it hurts and gives out. It hurts from the top of my knee cap to about mid thigh in one narrow straight line. I haven't done anything hard to strain it but it doesn't seem to get better. Ive blown out my knee before on the other leg and that didn't feel anything close to like this.

And the pain doesn't increase gradually either its either there or its not, no build up with the increased weight. Please help cause its starting to annoy me when i try to get out of a chair and all but collapse and have to hold myself up by the chair until the pain passes in a second or two. My opposite leg is getting tired of holding all the weight every time.

Thanks