I have sex before 4 month and I had period regularly after having sex but now after 3 period month now I had not 4th period... why..

There could be many different reason. We really can't tell you why, only a doctor can.

It could be that your period is just off. You could be pregnant. You could have a health issue that's preventing your period from coming.

How late are you? Have you tried taking a pregnancy test?

Periods can fluctuat from time to time. How old are you because it mostly happen through your teens. Sometimes a period will skip and sometimes it will be late. It happens to everyone. Just to be sure though take a test. If your period is more then 3 months late consult your doctor