Question about this girl I'm dating?

I met this girl at a concert and I asked if she had a man. She said,"No I only have a friend." So I took her word for since I have friends too,like Agnes, Agatha, Germaine, and Jack. So we started talkin', getttin' familiar and spendin' a lot of time so we could build a relationship. After this was established it was cool although she had to go back to school she gave me her number so I could stay in touch. However I called her room and a guy answered! I called again and asked "Yo who was that?" She said "Oh he's just a friend?" Should I be worried?