Knee pain?

I have been exercising with rowing ergometer now for almost two months consistently according to a gradually increasing interval exercise program meant for POTS, a health condition I have.

As the durations have increased, I've noticed I get knee pain. Or well, around third-halfway into the exercise (15-20min) my knees start feeling 'tired', which turns to mild ache by the end, and I am now also feeling it after exercising.

I understood at first this might be because knees are folding too much, and have after tried to keep them from not folding more than around 80°, but the ache still comes. It gives a bit of a relief if I let my legs fall out to the side (like ballet plies) or let them push completely together for some strokes.

Is there any other common posture faults or 'beginner' things I might be doing that might cause this? Or some precautions, knee brace? I'm 23 years old and don't have a lot of sport background so I'm a bit lost on what this might be from.