What Causes Nausea While On Depo Shots?

I was on the depo shot for years i got off of it for about a year and got pregnant with my first child she is now almost 10 months old and i have been back on the depo shot since she was a month old we use protection and i am on my shot i also do not have periods but the last few days i have been sick to my stomach all day long, could it be from the shot since my new one is coming up ? could it be pregnancy? i dont know why i would feel like this.