What Causes Inconsistencies In Menstruation?

Well about 6 weeks ago I had a regular period as I always have in the past, a week after I finished I had another period that lasted two weeks then two weeks after that I started a very light period which lasted a few days!! I just took a pregnancy test which came back negative, I havnt been excessively exercising and my diet is as normal! I got spots a few days before each of my periods which is normal for me and I am under some stress at work but nothing major as I believe this can be a factor!! I had been taking yasminelle for about two months prior to this happening but I took it very irregularly and have not taken it since...I'm just wondering what could be wrong and if I should be worried??