It is often difficult to distinguish a sinus infection from a cold or the flu. Often times acute sinusitis is caused by the common cold virus. When you have a cold you generally don't get a high fever but when you have sinusitis you can. A fever of over 101 degrees in adults is generally used to help determine whether you have the flu or a cold. This is where things get complicated and it is difficult to differentiate between a sinus infection and the flu. Although with colds you generally don't get fevers over 101, with any type of infection including a sinus infection caused by a cold you can get a fever over 101.
In order for a doctor to give you a diagnosis of sinusitis (sinus infection) they would have to give you a physical exam. Generally they will gather a history first and then check your nose and throat and feel for any area's of discomfort. They would look into your nasal passages for polyps or deformities that can trigger sinusitis. If the pressure and pain are extremely bad or it is a chronic condition they may want you to get a CT scan.
Basically it is impossible to say if you have a sinus infection or the flu without a physical exam. If you have a history of sinus infections, allergies, smoking or around people that smoke it is very possible that you do have a sinus issue. Even if you don't have any of these risk factors it is still possible. The best thing to do is to go see your doctor. If they do determine that your sinuses are infected by bacteria, you can get a prescription for an antibiotic. Antibiotics don't work on virus's such as a cold or flu however they do work on bacteria. If it is determined that the cause is viral and there is swelling the doctor may want you to take a small dose of oral steroids to reduce the swelling. Your doctor will evaluate you and determine if you need medication.
The general rule of thumb is a sore throat that lasts more than 2 days should be seen by a doctor. Other than that you can try over the counter allergy medication and if it helps then it is probably your sinuses. If it doesn't help that doesn't necessarily mean it is not your sinuses.
I am attaching 2 web-sites for you. They are about sinus infections and influenza.
I hope this information helps. If you have any further questions please let me know and I will be happy to answer them for you.
I hope you feel better soon. Thank you.