What Causes Irregular Menstruation?

I'm a 29-year-old woman with 2 natural-born children. For several years, I've experienced periods every 2 weeks, and they often last about 4 days. Sometimes it's just spotting, other times it's brown, or bright red on Day 1. About 2 years ago, I got an IUD (Merena). My irregular periods didn't go away. I took Yasmin for a year until my doctor recommended I discontinue it for cardiac safety. When on it, I was able to minimize my periods' length but it still came every two weeks in some form (spotting for 3 days). Now I'm on Errin (birth control) for the progesterone to treat migraines. I take it continuously for 3 months before taking 5 days off (placebo) to prompt a period. I'm still getting a period every 2 weeks. And now I'm getting an increase of cramps and bloating.

I had a laparoscopy before my kids were born to remove endometriosis, but the cells were too close to nerves, so little was accomplished. It has been at least 6 years since that.

Before I try yet another birth control pill, should I have the IUD removed? What is causing the irregular periods?