I normally have a heavy period, that lasts for about 6 days. I am on the 3rd day right now, and my flow suddenly went very light (it was heavy for the past two days, like normal). I'm 18 and have had my period since I was 10 and it's been regular and very predictable for the past year or so. What could cause the sudden change? I did get the TDAP (tetanus, diphtheria and pertussis) vaccine yesterday, that's the one thing I can think of that has been different with me lately, other than maybe stress from school... could either of those cause such a dramatic change?