On the face of it, there is little chance that you are pregnant, unless your last cycle was delayed and scanty and preceeded by sexual contact in the fortnight prior to it. If the last period was perfectly normal and so were the cycles prior to it, this could be a one off delay, and then the backache probably means your period is comming soon. A recent fullness of the abdomen may be coincidental, or a subjective sensation due to the overall heaviness that preceeds any cycle.