Hi welcome to Health care magic forum. It appears that your partners veginal PH might be acidic, due to some infection hence the irritation while peeing after inter course, for this both the partners should be treated completely .
I advise you to show her to a gynaecologist for diagnosis and treatment. the things you have described are symptoms of chronic U.T.I. caused by continuous exposure.
All your symptoms suggest some infection or malnutrition
. I advise you to consult a physician for diagnosis and treatment,You may need to have C.T.scan of the lower back along with other routine tests for confermation.
Wishing for a quick and complete recovery and thanks for calling.