Hi, So A Couple Of Days Ago, I Started To

hi, so a couple of days ago, I started to notice that my lower stomach was hurting really bad, like it was sore, like I pulled something, but it was like a burning pain everytime I stretched. then today, I noticed the my lower stomach was swelling up. I'm 13 years old, and I very skinny, and I'm not sure what's going on.