Hi, and thanks for your medical question on eHealth forum!
If you started your periods on March 10 instead of March 4th, and had a normal flow for normal duration, it is unlikely that you are pregnant. If you have back pain, lightheadedness, increased slep, then you can repeat the pregnancy test. A blood test would be more reliable than urine. Other possibilities are vaginal infection, stress, hormonal imbalance etc.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist and get this investigated. Hope this helps. Take care!