What Causes Migraines And Stomach Bloating While On Implanon?

Hello my fiancé has had implanon going on two years now. I ve been doing a lot of research and have seen tons of responses about how its very effective, and chances are one in a thousand. I have also seen that a lot of the symtons are close to the same as being pregnant; However this is still a scary situation. She has crazy migraines, somewhat of a skin problem such as pimples, eczema, or psoriasis. We cant really tell. Also weight gain around her stomach. we ve taken at least 3 pregnancy test and, all have come back (neg). Mind you, these symtoms started over the coarse of one week and, since they have started we have not yet taken a pregnancy test. We already have three kids, and cant really afford to take of another one. I need to know is this normal, and could my fiancé possibly be pregnant?