How Can Acute Knee Pain With Swelling Be Treated?

hi, i fell 3 years ago, injured rt knee (doc diagnosed-knee sprain)...i have had problems with it ever since. it never really felt "right". 3 wks ago, it began again, and hasn't stopped causing me EXCRUCIATING pain...tho i have not fallen on it again, or received any trauma to it. it stays swollen and i can't straighten it; it is locking now (didn't even do that when injured originally); it makes a very loud cracking sound when i try to bend it. i am miserable.