To rule out pregnancy
you conduct a pregnancy test at home for a quick assessment. Besides pain in your power back do have any other symptoms such as burning sensation during urination ? You need to look out for other symptoms such as itching, vaginal discharge
as you had unprocted sex.
If none of the above occur positive then u have a delayed period and the lower abdominal pain
could be related to the menstruation
Do check with your gynaecologists if your periods are delayed more than a week for a specialist opinion.