ligamint problem in my knees

About a year ago I was moving out of my apt. I was packing up the refrigrater taking things out and placing them behind me in a box. While I was doing this I felt my right knee dislocate or atleast thats what it felt like. I went to the emergency room the next mornig they told me it was a medial lateral ligament sprain. I repeated this injury 3 more times to me right knee. About a month ago i was walking to the car and my left knee did the same thing This time I did not seek medical atention. The next day I was sitting on the floor and my knee just looked as if it floped over and usually it flopes right back but this time it just stayed and when it did go back it swelled instantly. Now my quistion is can this be some type of disease or infection in my knee or with the ligaments. Since I wasnt doing anything the second time it happen I dont understand how it happend. If you can e-mail me some any information or anwers you might happen. Also some personal infor. I was 18 the first time it happend and now im 19 almost 20.