This is just a shot in the dark, but maybe it's possible that your Endometrium (uterine lining) isn't building up properly during your cycle? I know some women take baby aspirin everyday, because it helps get more blood flow to the uterus, and it supposedly helps their lining.
I'm pretty sure that if the lining of your uterus is really thin, then you will have shorter, lighter periods. It will also make it harder for a pregnancy to implant. I really hope I helped in some way. Good luck! Hope you figure everything out!
I can't give you an answer as to why it is happening but I have had the same thing for the past 3 months and I am definitely not pregnant. I am assuming it is just my body playing tricks on me because I want to be pregnant. All research etc that I have done shows nothing concerning so personally I have not gone to the dr about it.
Just to add .. have you been to the doctor for investigations as to why its taken so long?
Edit - Wow Tisha, thats really given me food for thought. Been trying 16 months now and took 2 years with my first. Never really thought of anything like that before. Sounds like that really makes sense though.