Pregnancy or something else?

I always get my period on time every month. My last period was jan 3. My period was expected around the 30th. My cycles are always 27 days apart. I haven't had my period yet. This has never happened. I thought I might be pregnant but I took a pregnancy test today and it was negative. I know stupid question, I just don't know why my period hasn't come yet, not that I am complaining. I will be making an appointment with my doctor if it does not come by the end of the week. I was just curious what you other ladies thought. I am 25 and haven't had any stress or excitement and I don't do anything more physical than usual.