The pain often indicates inflammation. You may need to rest longer to let it subside. Ice and ibuprofen are the standard for inflammation. I'd rather let it rest a bit more myself before popping pills.
Measure and compare the distance from the bottom of the pedal stroke to the top of the seat on both bikes. See if the new one is lower. Use a level (or eyeball) to check and compare the slope of the seats, should be fairly close to parallel to ground.
Get a bike computer with a cadence sensor. If your cadence drops below 80, shift to a lower gear.
There are some PT exercises to help build up and support the knee joint. I went to PT for a couple months for knee pain that started about 3 months after I resumed riding after many years and was ramping up.