I heard a pop in my knee while jumping on a trampoline. it was painful, there was/is no swelling . painful to walk on and feels tight. what is wrong?

Many possibilities!: From patellar misstracking, to tendinitis, to meniscal tear. Other possibilities. You did not say where the pain is being felt in the knee, so i cannot be more specific. I believe you should see your pcp! they will refer to to and orthopedist or rheumatology specialist if needed!

Mechanical injury: You can injured your knee but yo will need an orthopedic exam and probably MRI to accurately diagnose the issue if the pain persists.