What Would Cause Chills, Sleepiness, Couldn't Stay Awake, All In

What would cause chills, sleepiness, couldn't stay awake, all in and hour or two in the early evening, then high fever 102-103-102 emergency room, blood work OK urine OK Chest x ray OK. Temp went down to 101 and was discharged from emergency room at 1:45 AM Hospital thought dehydration and they said possible infection but didn't know what. OK now but would like to know what could have caused it.