Hello ma'am and welcome.
It is true that many women do not return a positive
pregnancy result until even 3 months into pregnancy, but that does not necessarily mean this is common and this is the same as is your case.
As your doctor has also mentioned, there are various causes that can result in symptoms and a presentation like yours (eg. stress, uterine abnormalities,
malnutrition, sudden loss of weight, birth control,
contraception, infections,
depression, etc.). If even one of these causes are present, then the symptoms you are experiencing could be because of that.
I would still recommend an
ultrasound which should certainly help in ruling out/confirming pregnancy, and once ruled out we can concentrate on the cause for your presentation and address it adequately.
Please consult another doctor if this one is not ready to conduct an ultrasound.
Hope this helps.
Best wishes.