When should i go to er for headache? i am 21 and for 3 days have had a headache behind eye on forehead. it comes on suddenly then fades away. it also hurts when i press on it. im scared of aneurysm.

Unlikely the cause: If this interferes with your ability to function you should get evaluated. Aneurysm would be one of the least likely causes at your age unless there is a family history or other known genetic condition. I would schedule a visit with your regular doc. If you go to the ER you will likely be triaged to the back of the line with the colds & rashes.