Pain in the knee a week after falling

Patient: I fell down the stairs a week ago. I had immediate pain, but the next day I felt better. I spent the next 3 days waitressing and only found my normal foot pain, no knee pain. Now 7 days after the fall I have pain in my knee when I try to hyperextend it straight. Bending is fine, actually feels good. But having it completely straight and standing is hard. If I walk with bent knees I’m okay, but I can’t have them straight. Does this seem serious or normal to flare up a week after?

Symptoms: Little swelling, pain while walking and straightening the knee