What causes irregular periods?

Hiii. Im start off saying i have an apt for June 2nd. So jus curious of experiences. I am a few days away from my 29th bday and my periods are mostly regular. Lately ive had half dollar sized bloodclots and cramps that feel like actual contractions. I jus thought that as im gettin older. Until I started my second period this month. Ive never ever had 2 periods one week apart. I hate this so much. Thank you for your time and input.