I think I can help! It has to do with the loosening of ligaments in the body during and after pregnancy.

I developed knee pain and weakness to the point where I felt my leg would collapse under me about 2 months after DS1 was born. Prior to pg, I had been a regular gym going and relatively active during pg. I got a referral to a specialist and had an expensive MRI done. The specialist said it was because I was fat (which I am) and was just degeneration of the knee joint and would never get better unless I lost a lot of weight. I left his office distraught. I didn't understand how "degeneration" could onset so suddenly. I went to my trusted GP who said to take fish oil tablets and see if it improves in 6 weeks or she could refer me for injections in the knee.

Sure enough my knee gradually improves (whether it was just time or the fish oil I don't know) and within about 2 months, it was completely fine (and I am still fat!). I was back at the gym, walking etc, everything completely fine.

After DS2 was born I had a bit of the same feeling in my knee, but less painful. It went away much quicker - perhaps because I got back to the gym more quickly this time...

Hope this helps!