I couldn't poop starting a few days ago and it's becoming unbearable?

I'm not sure how many days ago it was that it started, either 2 or 3, but every time I try to use the bathroom, nothing comes out and my butt starts to burn. I'm only 15 and used to suffer with constipation but it stopped when I was around 9 or 10. The burn is completely unbearable and I'm pretty much too scared to try to use the bathroom again. It'd kept me up from 2 am to 10 am and now it's becoming frustrating.