if you don't want to go to the ER, at least make a trip to urgent care ... if they really are black stools, melena(black stools) are not to be taken lightly ..

iron/pepto can cause 'em, but they are usually sign of an upper gastrointestinal bleed(high up like esophagus(food pipe)/stomach/small intestine ... gastritis/bleeding ulcer/varices(enlarged veins)/arteriovascular malformations/inflammatory changes(like IBD)/and yes cancer can black stools .. by the way have you been using a lot of aspirin or NSAIDs(ibuprofen/aleve and the likes) as these meds can also cause upper GI bleeds

and if you've been passing large quantities, it is possible that you are anemic

how this relates to your illness, can't be entirely sure, infectious process can do it but they tend to cause bright red blood per rectum(blood would be seen in the bowel) .. and how this relates to your painful bowel movements, heck my best guess is that you've been having enough bowel movements that you've caused superficial irritation of the anus, but you should be checked out

best of luck, and heck, take some time out for yourself