I had a 4day mens. after my bleeding we intercoursed(withrawal) 6 days later i got nauseous,crampings in my abdomen and lower back and spotting of blood for a day. now is 28th day of my cycle i felt pms(5days) but got no period. is this pregnancy?

Irregular menstrual periods are common. The fluctuations of estrogen and progesterone can be easily affected by stress, weight gain/loss, thyroid problems, and uterine issues such as adenomyosis or fibroids. If you are concerned about pregnancy, the only way to tell is to take a pregnancy test. These can be positive as early as the first day of a missed period.