Why am i 3 weeks behind schedule on my time when i havent have sex?

Why am i 3 weeks behind schedule on my time when i havent have sex?
Are you new to your length? Stress, hormones, over exercising can all be contributing factor. If you are new to your time of year, it will take around a year to start a regular cycle, not that in attendance is such a thing.
Stress, unbalanmced hormones... Go see a doctor, he/she would be the right creature to ask.
It could be your cycle is changing.
n00b hormones
It could be from stress.
stress, excessive exercise, anorexia
sometimes your cycle change... harsh change in weather, a stressful spell or an illness that you've be through lately may be the cause..
especially bulky medicine can enjoy such effects on your hormones!
relax...it happens more frequently that you can see in your mind`s eye..
however if it continues..visit a doctor! he/she would recommend you better than anyone!