What Can Cause Sudden Dizziness And Slurred Speech?

hello i have had a couple incidents that i cannot explain the cause of, the first when i was standing outside speaking to a couple family members and out lf no where became unbalanced and dizzy, my words slurred really bad the next thing that came out my mouth was just wierd noises i stumbled inside and had to lay down. The second time was monthes later, i stood up from sitting on my bed to get a glass of water from the bathroom. As i was filling the glass the same feeling came over me except i fell and hit my face and passed out, afterwards i got up cleaned up the mess, filled the glass again and as i was walking down the hallway it happened again and i was passed out for even longer and my roomate tried to wake me up but could not, i woke up on my own later. I was just wondering what could have caused this.