Moved to
Teen Pregnancy and Childcare Forum.
If I had to guess I'd say it's because each pill is different.
You're body is probably just adjusting to it is all.
I had to switch once because mine were not strong enough, I had mood swings but that was about it.
After a while, it got better.
If it concerns you then I suggest asking your doctor. This could be a normal side affect for this pill.