I have the same problem. I know it's probably from the pills, but I hate it too, especially in the summer when I want to go swimming. It's been 7 years since I was pregnant, so I know it's not related to that. I've been to 2 doctors many times, and had some tests but they still weren't able to tell me anything. They said the lining was thicker than normal, but there's nothing wrong, no fibroids or anything. Over the last few years, I have tried many different pills with the same results- Yasmin, Orthotricyclen, Loestrin 24, and Loestrin 28 to name a few. The "spotting" mid-cycle is usually lighter than my real period, but sometimes it lasts more days. And I get the bloating, cramping, headaches, etc twice a month too. However, the reason I continue to take the pills is to keep the PMS under control. Before, when I would get just one period, it was so bad I would be crying for hours for no reason and my husband wouldn't want me to be around my son like that. Now, it happens twice a month but not as severe. I haven't tried stopping the pills to see what happens, but eventually I will because there's more risks to it when I'm getting older.
Good luck, I hope you find an answer!