I think i have a sprained knee?!

i was roller skating yesterday, and i fell!. my friends tried helping me up and that made me fall directly on the front of my left knee, it is warm, swollen and black and blue on the side of the knee, and i didn't hit it there!? also pain standing up and walking, cant walk up steps at all!. should i go to a doctor!? is it possible to sprain a knee by landing on it very hard!? thanksWww@Answer-Health@Com

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