I am in need of some help/insight

I had my son 4months ago. I am breastfeeding but supplement with formula but my period came back two months ago and then this month I all of a sudden am late even though my last two period were right on time. I have no cramps but I am nauseated. This may make you think I'm pregnant well I took a test and it was negative. My fiancé and I have been using condoms and I know they are only 98% effective but if I got the negative on the test, why is my period late? And why am I not feeling well? It was the store brand test, I don't know if that matters.