I had sex with my boyfriend on the day of where my period should have started and now it should be my third day but m still not on my cycle please can you tell wats wrong?

Many things. You could just be having an off month, you could be pregnant, you could be stressed, sick, on medication, lost weight, gained weight, you could have an STD, and many many other possibilities.

There's no way to tell you why your period is late. It's too early to test for pregnancy, if it is stress, medication, etc. there's really no way to test to find out if that's the cause. If it's an STD, we can't diagnose that any more than we can pregnancy. So really, there's nothing anyone online can do to help.

Good luck.

Will agree, three days late is not really late, most perods very by a few days all the time. But it can be pregnancy, or infection, or STD, it is often worry and stress.

For not, you have to wait and see what happens, take a pregnancy test if you are 14 days late.