Kneecap pain with stairclimbing and deep squats...causes?

I’m a fairly active person and four months ago I added free weights to my routine when I had to give up running. Part of my routine is doing a series of step ups (the step is 36 inches high) in between weight sets. My problem started when the step up became too easy and I added 30bls of weights. Cardiovascularly I handled the extra weight no problem. But my second day of the new routine my kneecaps started to ache midway through one of the sets of step ups.

I stopped doing them with weights and then I had a week off as we went on holidays. But my knees seem to be getting worse and not better. In my regular life of walking, squatting and stair climbing the knee pain is quite pronounced.

I’m at the point now where I’ll go see my doctor if it doesn’t start getting better soon.

My question is: what the heck could be causing this? I know it is directly related to adding the 30lbs of weights to the step ups. Any insights?

EDIT TO ADD: Very little pain with walking as long as I don’t squat down.