Why is my knee clicking?

Patient: Im in the army So we exercise regularly My problem is about three weeks ago I was doing push ups in my room I was attempting to stand back up when my knee twisted and I had extreme Ever since then my knee has been clicking or popping as if its catching on something right underneath my knee cap Also when I try to run Ill get brief sharp stabbing pains on the underside of my knee also. And the clicking popping has become slightly painful I went to my doctor and he said I probably sprained it But its been three weeks and its getting worse