can you have a period and still be pregnant?

i had my last period about 4 days ago. i did have sex while ovulation time before my last period. i have been sick to my stomach and vomitting. i wake up at odd hours throwing up like 2 in the morning. i have aweful stomach cramps off and on. my best friend keeps telling me to take a test but i keep saying i had a period so im obviously not pregnant. could i be or maybe something different?