Why did beer give me a headache?

The reason you get a headache is because alcohol dehydrates you. Since you got a headache after only one and a half beers, it would suggest that you already were dehydrated before you started drinking. Even though you drank water and ate something afterwords it still takes time for a headache to go away. Alcohol has also been known to cause migraines too, so maybe you are sensitive to alcohol or the ingredients used in beer (hops, sugar, yeast, etc.)

I would also say there is a possibility that the beer was skunked.....If the beer has been exposed to room tempertures or higher or to lots of light or is just plain old (more than about 5-6 months) then it can very easily skunk and I have always gotten a headache from skunked beer. There is literally a moleculare chemical change/reaction and something resulting from the side-effect has always given me a headache as well as gas/heartburn

So, the question would be; is this common for you from drinking beer or other alcohol and if not did the beer smell or tast skunky? if you don't think it was skunked but also rarely drink then you could have an allergy to alcohol. Many people, inlcuding something like 50% of people with asian or indian american decent (and smaller numbers of many other races) have this allergy and the symptoms include a a hive like reaction and or headaches. But most likely the beer didn't sit you and very possibily because it was skunked

Drink another beer the headache will go away.

Your system is not immune to it

because you're still not getting used to it.