35 days since last period, negative hpt from doctor's office, cramps, what's going on?

Hi,
I'm 17 and I usually have a really normal cycle of about 25-28 days. The last time I got my period was 03/03/07 and I STILL haven't got it . This is causeing me to be very very worried that something is wrong. I have not had sexual intercourse with my bf but we do fool around (but we're very careful about everything). So I went to the doctors in the morning and they did a hpt and it was negative, so I don't know what's going on. I thought maybe cuz it was spring break that caused this because I was staying up till 4am everynight and wakeing up at 2pm and only eating like two meals a day bc of this. Also I did have a fight with my bf two days before I was suppose to get my period, I duno if that could cause it to be late too. Can anyone help me I'm really worried.