What Causes Spotting After Menstruation While On Contraceptive Pills?

Hi I'm having some spotting after finishing my period! Ive never missed a pill and always come on my period 3 days after I finish my pill! I didn't come on until day 4 which is unusual for me, also I had quite a light flow which is also unusual! I started my pill again and finished bleeding then a day later i started spotting and have a slight browny
discharge!! It's been happening for the past 3days! I normally take the pill at the same time each morning but was on holiday last week and took it at a different time each day! Could this be the cause? Or is it something else? Please help!