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You seem to have an infection, not just allergy and a possible sinusitis. The cause is most likely to be a virus.
Sinusitis occurs when there is inflammation in the sinus spaces in the head. You can continue Mucinex in the current dose. Robitussin is not really required. You can take a paiin medication for the headache - Something like Tylenol 2-3 times a day. Most important thing you should do is a steam inhalation. You should do a steam inhalation twice a day. You should inhale through the nose and exhale through the mouth. This will open up the sinuses and will reduce the inflammation.
Sometimes you might require an antibiotic for the infection, especially if the symptoms persists. Your Doctor can prescribe you one course of antibiotic in that case.
There is nothing to worry as the headache will resolve completely with treatement. The stomach upset could be due to the virus or from the medications. This should settle down on its own. No specific treatment is required.
I will keep this consult open so that you can get back to me any time.
Hope this helps
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