Why do I get these bad headaches?

Okay so I run all year round, (cross country, indoor track and outdoor track.) I take 2 weeks off after cross country and after indoor track. I also take 1 month of after outdoor track before I start my summer training for cross country. My problem is that during these breaks, I tend to get frequent headaches and sometimes they get pretty bad. I don't know why this is. Does anyone have any ideas? Help would be appreciated, thank you!