I have been a casual alcohol drinker for years (about 2 to 5 bottles per week) but recently whenever I take beer (not wine), I feel nausea?

Nausea. The nausea may be related to stomach lining damage, or gastritis. Some people are more sensitive to the alcohol related damage than others. Stopping the use of alcohol may reverse the process. But ulcers and gastritis from other foods, nicotine, stress is also possible. See your doctor for testing and medication if the nausea continues.
Beer Allergy. Dr. Albers is entirely right. But thinking outside the box, you might also have developed an outright food allergy or intolerance to barley, wheat, hops.....Since it sounds like you are drinking beer based on your question. I would 1st pursue gastritis, then if that isn't the answer see an allergist because we can test for allergies to these items in the alcohol.