My knee hurts whenever i bend it or any kind of movement. What are some possible causes?

I am a 25 year old female, i was walking up a slight hill. I have always had problems with my knee before but the pain always went away. Now the pain is contsant. I hurt it going up a very small hill in the park. I think i twisted it some how. How it hurts with any kind of movement. When i have my knee bent it feels like it is burning and hurts really bad when i try to unbend it. Sometimes my knee feels like the ligment or muscle on the top of my kneecap is pulled tight. I do not have a primary care doctor, i have no idea where to go, the only place i can think of is the ER. Any Ideas Will be Welcomed.