Bleeding in-between?

Kayla

I've been on bc for 2 years now and I take my pills religiously. My period ended January 15 and I still have 19 days to go until my next one (according to <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>). I'm on my second row of my pills. Today I noticed bleeding after sex, what would cause this?