Late period

If you only miss a month it could be either one of two scenarios - with the possibility of it being both. You could have missed it due to stress or other, and/or you could have missed it because your body - which is always changing no matter what your age - is reacting differently to the horomones that are being fed to it via pills.

 

now that I think of it, there are three scenarios, the third being that you are in fact carrying a child - but then you already said you weren't preggers