Hi Irishprincess. This is the hip flexor stretch that I have tried and found it hurts my knees for a couple of days afterwards. I think really I should say it is mostly my right knee as I damaged it a few years ago. What I damaged I have no idea as I did not go to the quacks.
I kneeled on a rock around the fish pond and had a very sharp pain in the side of my knee about where the lateral collateral ligaments are. The pain lasted for about 10 mins then was gone. Strangely for about 1 year every time I took a step my knee would go click click. The only time it would not click was coming down the stairs, but no pain.
Just over a year later I was metal detecting and even though I was conscious of not kneeling on a stone in field there must of been one under the surface and I found it!! Again a bad pain that lasted about a week and stopped me walking. As my job entailed a lot of walking I thought I would have to pack up work.
far forward a few years and hip flexor stretch as below takes me out for 2-3 days yet running has no effect on my knees.