Welcome to Health care Magic.
Based on the symptoms you have mentioned following are the possible causes
1. Urethritis- This often presents as frequent and painful micturition.
2. Prosatic hyperplasia- Commonest cause among men above 70 years.
3. Cystitis- Presents as Strangury, incomplete and frequent voiding which is painful. Sometimes some blood may be present in urine. Taking antibiotics will treat this condition
4. Diabetes mellitus- Frequent urination
is commonly seen in diabetics but pain suggests urinary tract infection
5. Defective spinchter mechanism.
As you have already been operated for prostrate the possibility of Prostatitis
can be ruled out.
Among other possibilities cystitis ( inflammation of Urinary bladder
) is the most probable diagnosis in your case.
But Diabetes with UTI has also to be ruled out. You have not mentioned whether you have diabetes or not.
Anyways get a complete urine analysis and Urine for culture and sensitivity done.
An ultrasound abdomen is to be done to rule out cystitis.
Based on the reports please see an Urologist
or a physician and discuss about the treatment options.
Wish you a speedy recovery.