Hello Thanks for writing to us with your health concern. Having sex on the last day of your period has a very low possibility of ending up in a pregnancy. However, there are cases reported, though the chances are very low. The possibility cannot be said to be zero altogether. Due to recent illness or fever ( chicken pox in your case ) also, abnormal spotting or menstrual delays can be observed. Also, stress due to any kind or recent fluctuations in your weight can lead to this. There is a surefire way to end your stress over the pregnancy issue. If you have experienced a delay of 5 days in your normal cycles, and your periods are always regular, then just take a pregnancy test to confirm that you are indeed not pregnant. That will resolve the issue. Also, if you wish to avoid pregnancy, please use a reliable birth control method or protection. All the best. Take care.