Hi Friend ..
Thanks for using Health care magic forum ..
I read your query and pinpoint black spots can be due to undigested particles that will look like mustard seeds.
I recommend you to get prescribed for Tablet Ofloxacin
And a probiotic
for 5 days and your health responds good like normal life ..
Drink plenty of water and energy drinks to stay hydrated .
In case it keeps on recurring go for a stool examination and if that has Pus cells go for a culture report and get treated accordingly by Gastroentrologist.
Hope this information is Helpful.