I have been climbing for more than 10 months now. I had been climbing regularly for 6 months before, unfortunately, I had a 2-3 months break from climbing. When I have gotten back to it now, I find developing jelly knees on my 3rd or 4th climb itself (lead). Because of this, not only am I not able to climb for as long as I used to, I am also not able to try harder routes because I am not able to trust my knees.
1) Any ideas how and why I might have developed these weak knees?
2) Can anyone recommend of any exercises that might be useful to strengthen the knees?