The chances of pregnancy occurring because of an isolated sexual act on the third day menstruation are very very remote. The egg release from the woman s ovary takes place 14 days prior to the next expected menses. Your low back pain could be due to muscle spasms and stress also. You are feeling sick, probably because of stress of anticipated pregnancy. As I told you, this isolated act is most unlikely to cause the pregnancy. Wait for the periods to come and that will answer your query. However, your menstrual cycles may be irregular at this early age. In such case, if you do not get your period at the expected time; i.e. one month after the previous one, better get checked from the physician. Remember, too much of the stress also may delay your menses.