I'd have it checked out; especially if there's swelling. This can lead to chronic knee problems if not treated correctly. Knee injuries can be the result of many things; previous trauma, osteoarthritis, patellar tendonitis, patellofemoral alignment problems, foot malignment, hip weakness....Strengthening the core muscles around the knees; hip, lower back, abdomen, quads is your overall best way to get rid of or prevent further knee trauma. If you're going to do cardio, stick to walking or the stationary bike if it doesn't bother you.