I react "backwards" to antihistamines... I took a hydroxyzine for a panic attack and was so wired I didn't think I'd ever sleep again. Of course if I take an Adderall, that makes me sleepy.
I’m not a doctor, but it's not uncommon for people with attention disorders to feel more tired when taking a stimulant. When it hits, your brain is finally not racing around and feels at peace. Just food for thought.
That's just how stimulants are said to work for people with ADHD. They don't have enough dopamine for proper functioning, so when they do get a boost of dopamine they are able to function more normally. Wired (hyperactive) -> tired. I'm not aware of antihistamines making people with ADHD wired, but I do know someone that used to get energetic when they took a benadryl.