I seem to be randomly bulking up?

I'm 15 years 8 months old- i have been doing the same exercises and eating the same amounts of food roughly a day, i randomly put on 1/2 a stone ( and i am sure it's not fat) , although my feet have grown a bit bigger over the last few months , i'm pretty sure this is not a growth spurt as i have already had a big one about 2 years ago , and in the last 2 years have grown maybe 5 cm only , so does any one know what is going on ? btw i have no history of any illness/ disease just incase there are symptoms matching this