With a 28-day cycle, your fertile period would fall around the 11th -15th day of your cycle. As such, you had intercourse during the safe period and there is no chance of conception and hence, no reason to worry. Yet, if you wish to get a pregnancy
test, the earliest possible would be to get a serum beta-hCG
titre about one week before your next period. This is almost always confirmatory. Hope this information is useful. Take care.