You can differentiate between two conditions. If you are pregnant, then there should be history of missed period ,at least one period missed. You will start to have naucea, breast tenderness, fatigue
. Here in this case, after one week of last period you can not decide on whether you are pregnant or not; as ovulation itself occurs on 12-14 days of starting previous cycle.
So as you are having back pain
radiating to sides, you can refer to general physician. After doing USG diagnosis can be made.