What Causes Irregular Periods Along With Light Vaginal Bleeding?

Hey. I have irregular periods. But so far this year I have only had one period which was January. I had no period February, and this month, march, I had one day of on off light bleeding on the same date my period started in January, which was the 12th. I also missed my period in December last year. But every month up until then I had my period. Just want an opinion on the cause to this. Thanks. Drew.