Suggest Treatment For Unbearable Pain In Knee

Hi I was at/ work where I wait tables and there are testers that go from a deck into the restaurant they are approx. 8 inches each -about a 2weeks ago I was coming up these steps and felt a twinge on the first step then another on the 2nd then all of a sudden it was like my leg wasn t there and I went straight down onto the floor and my knee cap at first I tried to walk it off and continued to work and have worked thru the pain since! Well today i was loading boxes into the car and as I came down the steps I heard a pop ( thought heard more than one but unsure) then a tearing or ripping sound as my leg buckled inward followed by unbearable pain shooting pain!! It s on ice swelling minamal can t stand on it without great pain stretching it out straight is painful as well! I have been reading about knee injury (lies) and it seems like there isnt a lot they can do? Do you think I should go to the ER. Or do you think it s an cal or plc please note that I have never had any knee issues and I land on it the first time and just as I was thinking that it could be getting better this happens any ideas!!!????