You can go to a walk in clinic like Rapid Care for about a $100 fee. Sometimes they have sample medications on hand for free, and if you do have a sinus infection it may be the cause of your intestinal issues as well. If you are running a fever it is important you find a way to get checked out.
A general rule of thumb in telling the difference between an infection and a virus is that usually one symptom will be predominate.
Just a severe sore throat, sinus issues, etc.
Also viral symptoms tend to unfold over a few days time.
Starts with body aches, then progresses to sore throat, then congestion, cough etc.
Bacterial infections tend to manifest with only one predominant symptom.
I am prone to sinus infections and they can cause stomach trouble too from swallowing the infected drainage. Often you will not have a lot of congestion with a sinus infection as it is blocking drainage and the nest of the infection is waaay up in your sinuses.
A nasty taste in your mouth, headache, fever, bloody, rusty or green drainage all suggest sinus infection.
They are nothing to mess around with, if left untreated they can penetrate the nasal membranes/cavities and cause a brain infection.
You really need to be seen by a doctor.