What Causes Nausea And Desire To Eat Sour Foods?

Hi,my last period was December 1st 2013. I dint get periods til day. But I did urine test in Jan, Feb and March as well. It turned -ve. I consulted doctor she suggested me to take Sysron -N and Lyto for 4 days. I complete the dose for 3 days. I am feeling vomitting and desire to eat sour foods like pickle. Please suggest if I have changes of pregnancy now?