What Could Sore Nipples With Bloated Stomach Suggest?

Hi im really scared im 21 and iv got a 4 year old boy iv been on the depo injection now for around 9 month but im really worried because i have really sore erect nipples and my tummy feels bloated and hard and if i put just a little bit of pressure on my tummy it feels really funny but iv done sevral home pregnancy test and they all come back negative what could be wrong with me