What's 'up'?

"Mama, a-ta-at" ("Mama, tumataas") ("Mama, it's up again")

Diego has been saying this (or experiencing this, rather) almost everyday, and sometimes 2-3 times a day! He even has this habit of getting it out and showing it to me. The first time he mentioned this was good for laughs, but now though sometimes I find it amusing, I'm a bit..what? concerned perhaps? I asked my mom about it and she couldn't remember any of my 3 brothers experiencing this when they were young (unless they didn't really mention it). I asked some of my friends. Though some say that it's as normal as can be, well, what causes it? I mean, is it the cold weather? Did he see something on tv? What? What?

My male friends would just laugh it off saying that I should be calm and be glad that my 3-year old is 100% hombre! Oh really??!?!! Geeeezzz! Men.