Bleeding/Spotting in between periods, what could this mean?

Bleeding/Spotting in between periods, what could this mean?-Lately I have been having some spotting/light bleeding in between my periods, and I am getting concerned. I am on birth control, tri cyclen lo, and have been for over 3 years. Any idea what it could be? I am taking my pills properly and on time consistently. Also sometimes my period is so so light, its barely there and it doesnt last long at all, its almost just a continuation of the spotting but a bit heavier. I could be stressed, does stress cause this? If it is not stress, what else could it be? 

I am not pregnant. I have not taken a test, but its been awhile since I last had sex, so yahhh. I would be really pregnant by now if that were the case! 

Any help would be appreciated!

  Stress can cause odd periods definitely. It affects your hormones which affects your period. If the bleeding / spotting is light I don't imagine it would be anything to worry about. Normally if there's something wrong, it will cause a heavy flow... however, I guess that isn't ALWAYS the case. You could have cysts, fibroids, etc. But what it sounds like to me is you're just fine. It's light and you aren't pregnant. If it gets heavier or more frequent go to the doctor and have him check it out :)