dizzy spells

i was having a mundane BM class when all of a sudden i started feeling dizzy. a kind of disorientated feeling where i couldn't walk a straight line and every sudden movement rendered me seeing stars in front of me. hmm.. how could this happen?? i only had this once before. that was because i got caught under the rain and forgot to wash my hair. but this time the sky shone clear and i really don't know what to do. i only know that dizziness is one of the worst ailments i have ever encountered. somebody help me!!!