Can you overdose on tylenol if you take 5? i usually take 4, but my headache was so bad i took 5?

I assume you was taking 5 tablets for the whole day because you said you used to take 4. If the total dose for the whole day is 5 tablets or 2.5 gm/day the dose is still safe if you do not have liver disease.

Dr Le has already commented on your Tylenol, but note you had a "bad" headache. If this becomes repetitive, then contact your doctor and explore reasons for headaches. Frequent intense headaches, such as migraine, when treated constantly with Tylenol have a tendency to become chronic daily more frequent headaches, so, work with your doctor if symptoms escalate.