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Hello and welcome I'm Dr Chip
Yes, you need to get back to the ER
While 95% of headaches are painful but not dangerous, the other 5% can be from leaking brain aneurysms and that might not have shown up at first with the ct scan you had
I would get an MRA of your brain--an angioMRI--to better check for a bleed
Please let me know if you need any more information