Hi,
Welcome to ehealthforum.
If your menstrual cycles are of 28 days, and last period started on 5th Jan, then you would be in your fertile period (time of egg release from ovary) at the time of your unprotected intercourse. Chances of pregnancy cannot be ruled out (since pre-cum is also known to contain sperms, and withdrawal/pull out method of birth control is associated with highest failure rates). Look out for symptoms suggestive of early pregnancy. The cramps that you experienced could be suggestive of pregnancy too. Consider visiting your doctor/gynecologist for examination and tests to confirm/rule out pregnancy.
Take care.