Why do one get head spin?? when one gets it first time ,does it recur again and again?

I claim to be healthy and i workout everyday and diet. Just recently my work got heavy and in the evening , my head started spinning even though i am enjoying my work however hectic is it.I controlled myself not to faint.And i vowed that this would never happen again.But then , after 2 days , my head again started spinning...i drink juices and eat fruits everyday and my weight is normal..Nevertheless , i got 2 head spins...why is it happening?