Is this normal 2 happen so fast? Need help!?

Well just an easy question.. I'm a 14 year old male and ever since I was a baby I've been very tall and skinny, I weighed about 110 and I'm 5'10. Very skinny. But ALL THE SUDDEN, over about the last 2 weeks, I honestly look like I took steroids!! I've gained about 15 pounds and I have a ton of more muscle, no fat. And I haven't evn been lifting or doing sports. The only thing I do is pullups, and Situps? So what could have happened? I mean, it's not a bad thing Its just very very weird... Thanks!