Welcome to healthcare magic.
itself can delay periods too so first of all dont worry.ideally even if you have no symptoms of pregnancy
its better to clear any doubt by doing urine pregnancy test.But in your case it seems less likely since you had period once after the intercourse , have a history of irregular periods
too .But weight gain, irregular menses, stress anyways indicate that you should visit your gynaecologist to rule out other problems like pcod or thyroid disorder
etc.hope I could help.