What's going on with me..

Hello.. My period is 2 months late at the least. I had a period in December on the 17th and it went off on the 22nd. So January rolls around and I have no period like none.. February comes and boom I have one.. It started on the 10th and ended on the 15th. Now here's march and nothing again, and now here is April and I haven't had one yet.. I been having pains in my stomach, feeling nauseous, headaches, and mood swings, I been eating a lot and being constipated and gassy... I took 4 pregnancy test and they all came out negative.. What should I do? And am I pregnant?