Birth Control

Have a serious question. I've been on the Depo Shot for over a year now and I know when you first start taking it, it can cause bad headaches and nausea but I mean I've been on it awhile and this has just started happening again I mean is this normal? I never started getting headaches and nausea like this until after the shot but I thought it was supposed to go away (which it did for a little while.)