3 weeks into doxycycline i have stabbing bladder pain when bladder is full. do i need complete urinalysis and urine culture? perhaps bacteria/protozoans/fungi not susceptible to doxy have grown unopposed?

Probably not...: That's a possible but unlikely explanation. The main infection as a result of broad spectrum antibiotics like doxycycline is localized yeast infection, usually involving the genitals, anal area, or mouth, rarely in the urinary tract. In any case, you should see your doctor about this; she may or may not recommend urinalysis and culture. Follow whatever her advice is. Good luck.

Why on doxy?: I'm curious: why have you been on doxy for 3wks? I'm assuming that it is either for acne or for prostate infection. If for acne, recommend switching to another antibiotic and seeing if doc will write you something to treat yeast (Candida). Diflucan (fluconazole) once or twice weekly for 2-4 weeks will help...compounded nystatin is better. If taking for prostate the prostate prob may be causing symptoms.