Bleeding for 1 month straight

So, sorry if this is a weird question. I have always had regular, 5 day periods all my life. However, last year, I had an experience in March of bleeding for 30 days straight. No pain or other menstrual symptoms, just light blood. Then the next month, my period went back to normal (5 days, typical period symptoms) and stayed normal the rest of the year. Then this year, I have just had the same thing happen again of 1 weird month long period (just blood, no symptoms) in April and now my period has gone back to normal (5 days, typical period symptoms).

Has anyone else experienced this? Having 11 regular periods and 1 really weird, long period every year. What causes this?