I do get some yellow stains on every single underpants i wear on, this has been on for a year and half now, the best i do is to change my set of underpants every month, its disgusting and tiring to be honest. its mostly the edge of private parts?

Put sone cotton balls or tissues in your underpants to absorb the fluids. Now no stains and you wash your pants and no need to buy new ones every month.

I would change them more often that every month. Could be drops of urine leaking. That will stain yellow. Sweat? Don't know how hot it it is in your part of Benin. Discuss with a local health care person