You can get cystitis after having sex, but that doesn’t mean you’ll get cystitis every time you have sex or that there aren’t any other causes.
Women often get cystitis after having sex because it can make it easier for bacteria to get into your urethra and move up to your bladder from there, causing a UTI.
If you use a diaphragm as your method of contraception this can also increase your chances of getting cystitis after sex.