It is possible that you are pregnant and modern pregnancy tests are able to confirm that fact within a few days of missing a period. The presence of back pain is not a reliable indicator that you could be pregnant. Sometimes when you stop taking the pill it can take some months for a regular pattern of menstruation and ovulation to be re-established. In some cases it may take three to six months for ovulation to take place after stopping the pill. The best way to resolve your current uncertainty is by having a pregnancy test performed.