To a small extent a difference in pupil size is normal, and may vary from left to right over time. However, if there is a larger discrepancy in pupil size it may indicate or warn of a neurological issue.
The determination of the causative of discrepancy in the size of pupil will need following:
1) A physical examination by a neurologist and eye specialist.
2) Fundus examination
3) CT or MRI of the brain and eye balls
Anisocoria (uneven pupils) is condition where one pupil is larger than the other. the causes can be trivial to some which will need medical intervention. Following are the causes;
1) Physiologic anisocoria; there is no vision loss or double vision, so nothing is to be done. The size discrepancy is usually 1 mm.
2) Horner syndrome; due to sympathetic dysfunction of the nervous system. There is associated eye lid fall. Will need further thorough investigations.
3) Third cranial nerve paralysis; blood vessel aneurysm or tumor may cause it.
4) Damage to Iris of eye; should be conformed by eye specialist.
6) Tonic pupil
So an examination is essential for pin pointing the cause.
Hope this helps.
Your follow up questions are welcome.
Your gastrointestinal problems may not be related to your unequal pupil. This may be, as you mentioned because of either iron or multivitamins. The C-section may be just a co-incidence, as medically they are difficult to correlate each other.
Since unequal pupil is quite a complicated phenomenon, it is difficult to sort out in online forum. At best it can guide you. The pupil size depends upon the effects of the autonomic nervous system and the iris muscle. So by a physician examination it is necessary to atleast tell what specialist to be consulted. An iris problem will need an eye specialist while autonomic nervous system problem would be dealt by a neurologist.
Light sensitivity, shoulder and arm tingling, tiredness, etc points towards neurological ailment and may be neurologist would be the best to start with. But as I mentioned, anisocoria can be just benign (nothing) or something serious, so you need to do something as soon as possible. With a small interaction with you, I feel your symptoms may be compounded by stress or anxiety also, so that part also needs to be taken care of.
Hope this helps you. Please feel free to ask your further queries.