I cant stop crying?!?

Ok so one morning my brother woke up at 6:30 and thought hey I'm not gonna wake up my sister so that she can be on time for school I'm gonna play on my playstation i was 45 minutes late to history on top of that I've got the history TAKS test coming up(a very big deal here in Texas) i was so mad i resolved to hide his game when i got home but when i got home ,when usually there's no one there my mom was home with my dad whom I usually only see on the weekends was there me and my mom had a fight when I tried to hide the game and i started crying but she seemed to understand when I explained why and we resolved the issue and my mom even made a joke that next time he makes us late we'd make him walk but when i went back into my room i burst into tears again and couldn't stop for a while I'm not sure if it had something to do with what happened that day or some other physiological reason but I just felt so horrible even though the the only thing that could've been making me upset was resolved I asked my friends and they said maybe it was a reaction to being late for cl but this was hardly my first really late tardy.have you ever been fine in every obvious way but were crying your eyes out what could cause this!? Its really bothering me.