Pain in muscle above knee

So recently I've been struggling with lunges. I was doing fine for the longest time but now I have to completely skip them. I get pain in my muscle right above my knee in the leg that goes BACK, not forward. It's gotten to where I can't even step off anything high unless I use my leg with the bad knee, otherwise the bending that happens to the bad leg when I step down with my good one creates pain. I've looked up videos on how to do proper lunges but it still hurts. It almost seems like a different problem. Can anyone help? Sorry if this didn't make sense.