Do you think I could be pregnant?

My period should have started the 28th of May, my last period was April 30th. I took 1 test 3 days after my missed period, negative and another 6 days after, negative. Both mid day testing. I do feel very nausea when I have not eaten, and if I eat too much. No soreness of the breast tho. I have a feeling in the lower part of my stomach, it's tight, like I did a million sit ups. I got a test to test first thing in the morning. I'm just worried that, I am not pregnant and something else could be the cause.