Some people are more sensitive to those types of meds than others so it's entirely possible. It's also possible to be experiencing a paradoxical reaction. For example Benadryl which is supposed to be very sedating actually makes me hyper so it makes sense to me that antihistamines in general could affect different people differently. You could try a different non-drowsy cough med that has different stuff/ingredients in it than the one you've been taking. I would ask a pharmacist about it instead of just looking at the store because lots of those cough meds contain the same ingredients even if they're different brands. They also tend to do more than treat coughs so lots of those meds have more than just a cough suppressant in them. It wouldn't hurt to get one that treats just the symptoms you need it to instead of also treating things you don't need.