Question about Periods

So I have a question about my period/birth control that I was hoping to get some advice on. A little bit of background on me: I am a 22 year old female with no std's. 

So I have been on the same birth control for about 3 1/2 years and it has been amazing. It's a continious one so I never get a period. And I never had any spotting! It's been great to me. Recently, however, I began spotting. This has been going on for about 2 months. Some days there is nothing, other days a panty liner works, and some days I have to wear a very light tampon. And the color ranges from dark brown to bright red. I haven't seen many clots... Maybe just one or two. It's just strange to me because it has been going on for so long and because it has never happened before. Even when I was off of birth control I never spotted between periods. 

Is it possible for your body to adapt to birth control and the birth control stop working? Or could this be a sign of something else? Could it just be stress? 

Any advice would be appreciated! 

Thank you!