Here you have symptoms like pain in wrist, hand and arm,
sharp pain in right side of head, as well you are a patient with POTS, and then you have got a lump at the base of your head right side suddenly. But let me tell you pots will not have this kind of symptoms what you have given so that is not the cause.
Here first thing I suggest is check your blood pressure since you are complaining of right side head shooting pain, later I suggest to visit a orthopedic doctor to have check up on
cervical spine x ray and pain in hand, wrist and arm level. If there is any problem at cervical spine then take treatment as per the suggestion by doctor as well also visit a physiotherapist for pain management through exercises, treatment as well guideline for postural correction.
You can also use hot and
cold pack for your painful area of hand wrist and arm. Or keep it at cervical spine if there is issue over there.
Now for the lump at the base of your head I suggest to use hot and cold pack there and observe for 2-3 days if it does not go then you need to check with ortho and neuro again to rule out the possible causes and take treatment as per that.
I am sure this will be helpful if you follow it step by step process.
Take care.