Cystitis is an infection in the urinary tract, the bacteria (from the intestine and commonly E Coli) creates an acidic environment in the bladder where they multipy very quickly. This makes your wee acidic hence the burning sensation when you wee.
Cystopurin is potassium citrate which neutralises the acid present thereby decreasing the acidity of the urine.
Sounds like it hasn't destroyed the bacteria properly and you could do with going back to the doctors. IIt could be something like Chlamydia or Gonorrhoea.
Have you been doing anything which could have reduced the effects at all such as vomiting or a large amount of vigorous sex.
Make sure you wipe from front to back after you've been to the toilet.
Wear 100% cotton underwear and avoid tights and tight clothing down there to allow the area to breathe.
Avoid using smellies down there or vaginal deodorants and such. A good standard of hygiene is required.
Drink lots of water and cranberry juice to flush it through your system and make sure you empty your bladder fully. Place yourself backwards on the toilet, so you lean against the wall. This posture is more suitable in securing a complete emptying of the bladder. Try to have a wee at least every three hours.
Go for a wee as soon as possible after sex.
It can be common in post menopausal and pregnant women and after radiation therapy if any of these apply.
Hope it clears up soon xxx