Confused if it's 8 weeks or 5weeks

Hi mga momsh! Based sa due date calcu at apps, I was already 8wks preg. But the OB said it's just 4-5wks. The 1st day of my last mens was Feb 4. Magpapacheck up sana ako kasi may bleeding when I pee at masakit balakang ko baka may problem. Kaso sabi nya, wlang OPD at di pa daw makikita naman sa ultrasound kasi 4-5wks palang. The OB advised me to drink duphaston and take a urinalysis nalang. Am I just 4-5 wks pregnant at hindi pa ba talaga pwede pa ultrasound to know it's heartbeat?