I've been on the combined pill for 8 months and never had breakthrough bleeding till now. Why? Pregnant?

The pill i'm on is called Cileste. The bleeding started on Friday 27th, I took my last pill in the pack today, Monday 30th. Also, it seems to be more than just spotting. I'm anxious because obv i should have my 'period' soon and i don't want to mistake this bleeding for my actual period and then find that i'm pregnant later.