lower left side and back pain

I began taking ferruous sulfate(325 mg) three times a day 10 days ago.  I am not constipated but my stools are little single pellets. My left lower side and back are aching very badly. The pain is constant.  What can I do?  The ferrous sulfate was prescribed hair loss; I've loss my left eyebrow and hair on the top of my head is thinning.  My dermatologist thinks it may be because my iron level is low.  Also, I am allergic to sulfur.  Should I take ferrous sulfate? One more thing even before I began to take the ferrous sulfate my eye lids have become dark; this began to happen a couple of years ago but my dermatologist is unconcerned.  I am very concerned that it may signal some underlying issue.  Please help me.