Implantation bleeding only lasts for a day or so and is light/spotting. This occurs around or just before your period.
This is because after conception it takes about 6-10 days for the embryo to travel down the fallopian tubes and embed/implant in the uterine lining.
Also for this reason implantation bleeding only happens once - not every month.
Implantation bleeding is not common and not many women experience it.
Your best idea is to go back to your doctor and get a complete health check. Something is causing your weight gain, nausea etc etc.
But negative pee and blood tests and an ultrasound pretty definately confirm your negative result.
If you were too "gassy" it shouldn't make a difference to the ultrasound as they would then use a transvaginal probe instead of the abdominal one or call you back at another date.