Always take medication according to the directions because an overdose could make you sick or kill you.

What you took, as someone else pointed out is DPH or Diphenhydramine. It's often used in allergy, cold, or sleep medication. It isn't for headaches and unless you were drinking a lot of water with it, it probably just dehydrated you a bit and made your headache worse. DPH is also a deliriant at higher doses so that is what made you hallucinate. Some people do this to trip, but it isn't considered very fun or enjoyable compared to most drugs. Most people I knew who abused it were depressed, and used it as a form of self harm.

You're probably lucky that it wasn't headache medication actually if you're just taking a bunch and hoping to feel better. If the bottle has a lot of Tylenol (acetaminophen) in it you may have needed to go to the hospital to have your stomach pumped. Some drugs/active ingredients are more dangerous than others.