Hi, I'll Be 14 In November And About A Day

Hi, I'll be 14 in November and about a day ago i realized below the crease of my left elbow was swelling. I started to freaking out because this has never happened to me before. So it's a day later now and I've been trying to figure out what it is and I found a lump a little bigger than a quarter in the crease of my elbow. by now I'm totally freaking out and I can only feel it when I extend my arm out and I'm stretching it. it is not visible to the eye and it is under my skin. but when i bend my arm it seems to disappear. i am so scared and i haven't told my parents yet. WHAT COULD THIS BE AND IS IT LIFE THREATENING