Yes, it is very hard on knees pushing a big gear. Rest until the pain is gone, then ride more. Don't ride if it still hurts or it will take way way longer to heal. Unless you tore something, then you need a MRI, not just X-ray.
Can you screw up your knees riding SS bike uphill?
yes ! I did. Some people say it makes you stronger but to me that is the dark side of single speeding.
I built up a single speed and on my first couple of rides I rode up a great big steep hill right by my house.
Then my knees started clicking ever time I walked up or down steps. It got worse so I quit riding the single speed. I was upset that my single speed bike might have messed me up so that I couldn't ride bikes at all.
I was really scared when my knees did not get better - they clicked (and hurt) for over three months!!
Finally they got better - I sold my single speed as fast as I could!
Riding my normal geared bikes does not hurt my knees or make them click.