Why do I keep getting stronger when I don't work out or play sports?

Me and my buddy are 14 we are playing video games at my house and he says "Hey dude have you been working out your starting to look alot skinier"I said no and after a couple hours my friend leaves and I decide to look in the mirror and I see I do look alot skinier and a bit more muscler and I think to myself "I don't even workout but mmh who cares"To me I thought okay this is good but back then which was a month ago I really didn't care but now I look in the mirror and see myself with abs,strong arms,and everything you would see a varsity athlete have but the thing is I don't work out or play many sports I was 6 ft but weighed about 160 pounds but looking at myself know it really doesn't make since how that could could change to this so fast and with no effort and my mind power has been getting better too in two weeks I shot from 11th best in class to 1st best in my math class and I have been able to work out all these tough equations in my head so much faster and all my grades in my other classes have shot up to the best in the class too and I have been able to focus so much more.So what the hell is happening to me I told my friends what is going on and my parents they say maybe I am just matureing are they right?Because I tried to find info on matureing and and from what I found it doesn't say anything about whats happening to me are my parents right or is something freaky happening to me because none of this seems normal