I had this issue PP. I immediately noticed it bending to put LO down or picking her up from her floor activity mat. I also tried some light yoga and stretches starting about 1.5 weeks after delivery and my knees, particularly one side, hurt a lot! It does get better as you build your strength back up....I just had to slow my pace and plans for exercise down. I'm 2.5 months PP now and it is vastly better. Your joints can become more relaxed during the last part of pregnancy (great for pelvis, not so for knees!) so it takes time to feel normal again. I did a few cross training workouts after my 6 weeks which had a lot of lunges and leg muscle circuits. I was extremely sore for a few days but the strength is much improved. Hope it gets better!
I had awful joint pain until almost 5 months pp. The worst was the carpal tunnel, but also suffered in the knees and ankles. I was getting so fed up that nothing was working consistently. I invested in a blood flow stimulator wrap and it did help but the main thing was time. The pains are inflammation brought on by hormones. It will go away...eventually. If interested in the wraps I used: https://kingbrand.com/BFST-Home.php?REF=34PV16.280
Re: PP Knee Pain
they finally went away.