Help negative hospital test???

Hello everyone so I went into hospital last night with abdominal pain, anyway the doctor didn’t suspect ectopic but he dipped my urine and it came back negative he told me sorry your not pregnant, then said I’ll check your bloods, my bloods were 752hcg I’m roughly around 4weeks 5 days, how come there urine test was negative, I’ve got a scan and more bloods on Tuesday as he can’t guarantee  its not ectopic or a chemical without the scan. I just don’t understand. Thankyou.