1997 Nissan Truck My truck overhated and i don't know why..

Hi i have a 1997 nissan pickup xe. My truck overheated at work last night and i have no idea why... i replaced air filter a week ago, topped off coolant two days ago, topped off antifreeze three days ago, and it was fine until tonight. My question is what happened that i missed and what can i do to fix it? Thank you very much!