What Is The Irregular Spotting,headaches And Nausea?

Im on the depo jab i have to go back on the 13th may to get my 3rd jab, About 3 weeks ago I i started spotting its everyday but its not all day it happens for about an hour or two then stops then happens again, I've been feeling sick, having head aches, feeling faint, tired all the time and feeling quite hot i did a pregnancy test about 2 weeks ago but it come back negative, ive never had this problem with the depo jab.