Started A Few Days With Pain, Stiffness, Behind, Left Side

Started a few days with pain, stiffness, behind, left side of, right leg, pain stiffness has got worse. Today 4th day, walking downstairs, felt sudden, sharp pain behind knee. Can't use leg, or and
, and whole leg very painful, swollen and useless. What shall I do? Shall I paramedics, as I live alone. Normally fit, goes to gym, age 65,

Bitte