Had my period normal flow for one day and now the second day is very light. what could cause this?

Variation: Periods can vary for many reasons, we usually don't know why. If you had an anovulatory cycle (one where an egg wasn't released) this can cause a change in your normal cycle length and your bleeding during the period. Stress and overall health can also play a role. Since you are young, your hormonal cycle may still be developing and you can have occasional odd cycles.