This time of year can be confusing when it comes to pregnancy and allergy symptoms which aren't that different in the first trimester!
I'm not sure when you had your ultrasound done, but if it was recently, say, within the last week, then it wouldn't have shown you if you were pregnant or not since it was confirmed that you were ovulating. If it's been at least ten days since your ultrasound, it won't hurt to pick up an early detection pregnancy test, like a First Response Early Response and take one with your first pee of the day.
If it hasn't been that long since your ultrasound, wait until the day your period is due and take a test then - the result should be accurate whether it's positive or negative.
Now about those pesky symptoms! Yes, dizziness, sinus pressure and pain, headaches and gagging/nausea are not uncommon in early pregnancy for many of us. That's because when you become pregnant your body works really hard to create extra blood for the baby and in doing so other weird things happen like sinus congestion which can leak into the back of your throat and upset your gag reflex, causing gagging and nausea. So fun!
The other thing that can cause these symptoms that isn't uncommon in the spring is seasonal allergies which I'm personally dealing with as I type this. So, if your test comes back negative and you're still dealing with these symptoms, especially this dizziness and sinus pressure/headaches let your doctor know and ask if the cause could be allergies and what type of allergy relief they recommend for you. Taking my over the counter allergy tab every day this time of year really helps me to function though I still have bouts of dizziness and nausea thanks to the post nasal drip.