I hurt my knee running and it has a little swollen bump underneath the knee? Is this serious!? It's been four days and I haven't stopped my activities

Patellar tendon? I am assuming "underneath" means just below the knee cap and "hurt my knee running" means pain after running and not a specific injury. This is your patellar tendon. You may have tendonitis. I would rec activity modification, nsaid's and ice. If not better in a few days, then ortho doc eval.
Probably not. Not sure how u hurt your knee or if u fell on it or bumped it... Sounds like u have prepatellar or infrapatellar bursitis. Common and often after a fall or just by getting bumped on your knee. Ice, NSAID may help if swollen. No other specific treatment is necessary unless it becomes infected or recurrent (and painful)... Old term "housemaid's knee". See md if persists and for definitive dx. Gl!