I've had this happen a few times for whatever reason.It went away after a couple of easy days.I've actually had to lower my seat a tiny bit at times to help it along.

My pain is mostly all over,old man pushing too hard for too long.

Normally pain in the back of the knee means the seat is too high...pain in the front means the seat is too low...pain all over your knee,the gearing is too high or pushing too hard in a high gear

Pain in the sides of your knees is from clipless pedals.

I would take it easy for a couple days(rest or low miles),lower the seat 1/4-1/2 inch and see what happens.If it's something simple,it should feel better in a day or so.Don't keep riding if the pain doesn't get better.