as you are having history of unprotected intercourse
though you practiced withdrawal method, sometimes pre-ejaculate
fluid can contain sperms,
so the chance of pregnancy cannot be completely ruled out,
you better check by blood test
for pregnancy which is more sensitive and can detect low level of HCG also.
if it comes negative then after consulting your doctor you go for ultrasound
and hormonal evaluation to detect the cause for your menstrual irregularity.