Even if you’ve experienced it for years, the sight of blood marks in your undies generally isn’t a welcome one — especially when you know for sure they’re not a result of your period. Spotting between periods, also known as breakthrough bleeding, is relatively common, particularly in women between the ages of 20 and 40. However, it can signal a number of underlying causes — here are five of them. 5 possible reasons why you're spotting between periods