Could I really be pregnant?

I left the doctors this morning finding out I’m positive from a urine test. I personally do not believe them because when I went and got a test after the doctors it was negative. Is there a reason why it was negative for me? My period is 5 days late, it came at the end of June and it being late a few days is nothing new. The only thing I been feeling is nauseous and I just feel like I been feeling like that more since I been told. but I’ve had a weird feeling something been off. I was nauseous also in mid July but I assume tummy bug bc my 2 yo was sick also.