You don't mention where the pain is in your right knee. It might be helpful in identifying what the problem might be if you can tell us whether it it on the outside, at the kneecap, or in behind.

A slipping seat post might appear to be the immediate cause. In this case, the pain may well be at the front. How have you fixed the seat post so it doesn't slip? Also, do you ride with flat pedals, or straps and clips, or clipless pedals? And what sort of shoes do you wear?

Otherwise, have you had the bike fitted to you? Use lower gears so you spin the pedals in, say, the 80-90rpm range.