Allergies cause a wide range of symptoms, from the common problems that everyone has heard of such as a runny nose, nasal congestion, sneezing; watery, itchy, red eyes; and itchy ears, to less obvious problems like a tickle in the throat, cough, asthma, wheezing, shortness of breath, hives, eczema, bee sting reactions, and so forth. Fatigue is also common in allergy season. The older antihistamines which cause drowsiness didn't help with fatigue but now the newer antihistamines generally don't cause drowsiness.
People develop allergy problems because of an overactive immune system in which certain gamma globulin's are overproduced by the body. Gamma globulin's are normally present to fight bacterial infections. In the case of allergies, one of the gamma globulin's (called IgE) functions abnormally and views the allergens (e.g. cat dander) as being a foreign bacteria - so the IgE attempts to "kill" it. When IgE attaches to the pollen, a barrage of chemicals including histamine are released into the body from cells called Mast cells, and as a result patients are left with allergic symptoms.
Most people think that allergists only deal with allergy problems, but as you can see from the information contained in this website, we deal with a lot of other problems including sinusitis, asthma, hives, and so forth. Just because you go to see an allergist, it doesn't mean that you necessarily will get treated just for "allergy" problems, especially if you have other kinds of problems that are bothering you, like sinusitis. It also doesn't necessarily mean that you'll get allergy tested. There are many other things that allergists can do to help you in addition to doing allergy testing.