preganancy test was positive but mensis came after 11 days of due date, reason

my mensis date was 21 but mensis didnt came, i did the preganancy test at home ,it came positive but one line was bright and 2nd line was faint it means that result was positive. next day i
gave my urine to lab test. result was positive. but at 2nd of next month(after 11 days) i got up in the morning went to bath and mensis came. i consulted a doctor that day she performed the ultrasound and said u r not preganant. i dont understand whether i was preganant or not. because last month mensis came after 10 days of due date but at that time preganancy test was negative. me and my husband are worried please help.