Irregular periods for no valid reason? Why??

I've had irregular periods for most of my life, I was on birth control for 8 years and was still irregular but not as much, I got off BC 5 months ago and have only had one period, which was last month. Now this month I'm late and probably not going to start. I've been tested for psos and thyroid problems, I'm not very stressed and have a relaxing life, I workout 6 days a week and am slowly losing weight to get in a normal bmi range, so if everything is fine then why am I so irregular? Is there anything to fix it?