More information is necessary to have an idea what’s up. When you say the backs of your knees do you mean the tissues (ie muscle and ligaments) or does it feel like something else? Is it more on the sides or middle? Does it hurt when you press on it? Is there any swelling? My suggestion would be to talk to your coach about it. He/she maybe able to feed you some insight based off of what they see.
If it feels like soft tissue the general rx will be smash north and south of the tissues and try to feed some slack into the system. Work the entire length of the leg with smashing and stretch the calf, hamstring and quad. Voodoo flossing works great here but a foam roller/ lacrosse ball can get the job done too. Get some low level movement (ie walking) in throughout your day and make sure you’re drinking plenty of water (with a little salt).
Also beware of adaptation errors. It’s probably not a good idea to smoke your legs with step ups and squats then sit in a chair for 8 hours without trying to do some maintenance. Not saying that’s what happened but it does happen a lot.
The other concern is swelling causing the ache. Compression, distraction/ flexion gapping, and low levle movement would be advised.