You'll have to make the decision about the ER on your own, but if you're able to contact a doctor he or she would be the best person to ask. If you go to a certain physician regularly you might be able to call him or her up and get through to someone on call. Or you could call the hospital and ask them.
Having a headache that's effected by your posture is common. If you've been lying down, sitting or standing up causes a drop in blood pressure as blood pools in your lower extremities because of gravity. Your body responds by constricting blood vessels (and increasing your heart rate) to quickly maintain regular circulation. We know that the blood vessels in our brains play a major role in the experience of a headache. So it stands to reason that a change in blood pressure, blood vessel constriction or dilation, as well as other considerations can be headache triggers. What I mean to say is that your experience doesn't sound particularly abnormal.
You should be thinking about whether you've had headaches in the past and how this one compares in its severity and overall effect on your body. If your only essential symptom is head discomfort then you probably need not worry so long as it stays at a tolerable level. If it significantly worsens or persists for an abnormally long period of time then you should seek medical care/evaluation. You really are the best judge as to how much your body is being affected by this problem. Try to evaluate your overall level of distress aside from the worry that you're experiencing due to your abnormal (for you) symptoms. Again, if you can, there's no harm in asking the advice of a doctor.
DX: NDPH, Recovered CRPS
RX: Lamictal, Methylphenidate, Clonazepam, Wellbutrin XL, Namenda, Oxycontin, Oxycodone, Magnesium sup.
PRN: Ketamine nasal spray, Celebrex, Migranal, Haloperidol, Zyprexa, Lodine, Zofran, Phenergan, Thorazine, DHE IM, Droperidol IM, Toradol, Reglan, Ambien CR, Provigil, and all triptans