It is not uncommon to experience symptoms such as severe back pain, tiredness, bloating, gas, and mood swings soon after the day of your period if you have tested positive for pregnancy. These symptoms can be caused by hormonal changes in your body as well as the physical changes that occur during early pregnancy. However, it is always a good idea to consult with your healthcare provider to rule out any other potential causes or complications. They will be able to provide you with personalized advice and guidance based on your specific situation.