What Causes Brown Colored Spotting While On Contraceptive Pill?

I was recently on vacation, on my way back home, two days of flying, I started spotting in the airport which I only noticed went I went to the bathroom. This continued on for two days. It was very light, brownish in color, not enough to require a pad. I also had the stomach flu during this time so I'm wondering if that could be why i was spotting? I'm on the pill but I did not miss any pills or take them late.