Left lower back hurts?

So i was on my way back to my room to watch some tv and eat some food and all of a sudden it felt like my left lower back was falling a little bit and the pain sudden came on. I couldnt sit up straight so i layed down, which i dont feel any pain while doing so. i eventually had to get up and do some homework then my back seemed like it cracked a little bit and it feels a little bit better but not all the way. I tried cracking my back but nothing happend.

This has happend before when i woke up for school one morning but i thought it was because i slept on my back wrong. im not sure if it was in the same spot but i do know that it was in my lower back. I cant bend over without falling weak to my knees. I eventually went to school then came home because i couldnt focus nor could i sit still in my desk.

I honestly dont know whats wrong or what caused it? Im a 17 year old girl.

Please help my lower back hurts, i cant even sit up straight all i can do is lay down and watch t.v. But i need to get things done before the day is over :(