The back of my neck is also stiff below the back of my head
Do I take more medicine or is this a side effect after taking the medicine
Sumatriptan is a Migraine abortive medication that stops the Migrainous process. These are to be taken at the first sign of a Migraine attack, but not more than tow to three days a week. Taking triptans, ergotamines or any kind of pain medication more than two or three days a week can make matters worse by causing medication overuse headache (MOH), aka rebound. See Medication Overuse Headache - When the Remedy Backfires for more information on this.
Muscle stiffness may be a potentially serious side effect, please contact your doctor and/or pharmacist, I wouldn't use a triptan five days in a row because of the potential for MOH. There are other alternatives.
Anyone who has three or more Migraines a month needs to be talking with their doctor about prevention. There's growing evidence that Migraine is a progressive brain disease. A recent study showed that Migraines can cause brain damage, and that people with three or more Migraines a month are more susceptible to this damage. For more information, see Is Migraine a Progressive Brain Disease? and Yes, Migraines Can Cause Brain Damage.
I hope this helps,
I don't know if the pain is a side effect from the medicine but you might want to check with your doctor. I'm not totally sure what sumatriptan is, but if it is like other triptan abortives you are not supposed to take it 5 times a week. You could be creating a rebound pattern for yourself.