Why does it hurt like hell ?

Ok so I was brushing and I have a small mouth and one night at the last tooth on my left side hurts like on the gum and I brush really well but my dentist says to brush the back teeth. Which I do I try. But the brush of my tooth brush hit the gum on the left of my last tooth and it really hurts like hell I brushed a bit harder to get it to bleed but that didn't work and it's really sore. What is wrong and I told my mom but she thinks it's a canker sore but there isn't anything there and I took anvil for the pain and I'm wearing braces what should I do help please ASAP! I'm scared do I need surgery for this is it a bad toothache ? Help!