Tanrums

I'm watching my room mates 3 year old son while he is out getting his paycheck. His sons name is Kaiden and he is the most well behaved toddler EVER!! But he just scared the hell out of me. He threw a tantrum. He was eating his breakfast and got up from the table so I told him to go sit back down and finish eating. He went back and sat down and started whining so I went into the dining room to see what was wrong and his bowl was eating he had finished. So I told him "why didn't you tell me you were done you can go play if you are finished" and he started crying so I got him up from the table and trued to calm him and get him to play and he just started crying harder so I was like ok come in the living room and sit in your favorite ninja turtle chair and calm down then he starts to throw the tantrum banging his head against the wall and refused to sit and started spitting everywhere so I gave him a small pop on his tush and he started SCREAMING!! I sat him down and talked to him and cleaned up all the spit and he calmed down so now he is playing around and happy as usual.

This is a kid who NEVER throws tantrums (this is his first ever!!) He is normally so well behaved and he listens so well but today something is different. He hasn't been listening all day and now this. After he was playing for a few minutes I started checking his temp and his ears and trying to see if maybe he had a tummy ache because there just had to be something wrong for him to be acting this way but he's fine. Wtf?! How do we prevent this from happening again?? Why is such a happy well behaved child acting out this way??