Urinary Tract Infection and Cystitis: Bulldogs are prone to UTI's (Urinary Tract Infections) and bitches are prone to Cystitis - especially when in season. A drop of Cider Apple Vinegar in the drinking water will help. If the condition continues you must consult your vet as a course of antibiotics will be required or in the worse cases xrays to rule out bladder stones or crystals.

Cystitis is an infection of the bladder, but the term is often used indiscriminately and covers a range of infections and irritations in the lower urinary system. It causes burning sensations during urination and a frequent need to urinate.

Pyometra: In it's simplest terms, pyometra is an infection of the uterus.
Infection in the lining of the uterus is established as a result of hormonal changes. Following estrus(heat), progesterone levels remail elevated for 8-10 weeks and thicken the lining of the uterus in preperation for pregnancy. If pregancy doesn't occur for several estrus cycles, the lining continues to thicken until cysts form within it. The thickened, cystic lining secretes fluids that are an ideal environment in which bacteria can grow. Additionally, high progesterone levels inhibit the muscles in the wall of the uterus to contract as well as lowering the dogs immune system in preperation for pregnancy.
Pyometra may occur in young to middle-aged dogs, though it is most common in older dogs. After many years and maybe as soon 2 to 3 estrus cycles without pregnancy, the uterine wall undergoes the changes that promote the disease.
If your dog is NOT for breeding, please consider spaying her. Heat cycles are hard on the female, the risks of unwanted pregnancies from stray local dogs, your bitch escaping(perfectly normal behaviour) and the risk of pymotra. I think it is something to be seriously considered and advised about. Remember a dog with pyometra will be spayed and the cost is about double that of a healthy dog. Also pyometra can kill.

Vaginitis: A vaginal discharge usually yellow or green means infection but there are 4 colours, maybe 5 from my experience.
Clear happens during pregnancy starting 2 to 3 weeks before birth. Dark red after birth for 6 weeks or more clearing out the left overs. Red during her season(blood). Then there is brown which I cant remember. Vaginitis is easily treatable with antibiotics.

Thrush: is another vaginal problem dogs can suffer from. My vet said yoghurt in the diet helps but maybe there is better treatments.