What Does Absence Of Period When Undergoing Diet And Exercise Regimen Indicate?

Hi I'm really unsure what my body has been going through lately jan 25th I got my period I started a diet in feb for about three weeks including exercise every night to, the last week in feb I started slacking off on my diet and exercise and I noticed I didn't get a period in February it is now March 10 and I still haven't received a period and I have taken 3 pregnancy test all negative and btw I'm 5'7 around 190lbs and my longest cycle averages 40 days I have had sex several times but we used protection I'm just trying to figure out what's going on with my body?