What Causes Delayed Menstrual Cycle When On Medication?

I am 30, changed my pill a week before my period. I am suppose to be having my period this week, but it is very light and dark, today is the forth day. I feel bloated. 3 weeks back I had throat infection and it was treated with amoxicilin. I had vaginal thrush two weeks back. I went to the clinic to do pregnancy test yesterday and it was negative. I have never missed the pill for the pass 6 months and I am not trying to have a baby