"Headache" is a countable word. It true a person can only experience one headache at a time. However, many people can collectively suffer "headaches". Also, a single person can suffer many "headaches" in sequence. "Headache" is also used as an idiom to mean "unpleasant circumstances." (For example, "Paying my taxes on time is a headache.") If a person is experiencing more than one unpleasant circumstance, that person may say "I have too many 'headaches' right now." A person cannot literally have more than one headache at a time, but when "headache" is used as an idiom, it is still countable.

yeah, i know, the problem was i thought "ache" to be an abstract noun, thus uncountable

Uncountable nouns don't use 'a' or 'an'. This is because you can't count them. For example, advice is an uncountable noun. You can't say "he gave me an advice", but you can say "he gave me some advice", or "he gave me a piece of advice".