My girlfriend complains about pains when she pees after we have sex, either right after we have sex or the morning after. is that something bad?

Honeymoon cystitis: Bladder or uretrhal inflammation or even infection can occur in some women after sex, especially if prolonged. The symptoms are those of a urinary tract infection. An actual UTI should be ruled out and then a single dose of antibiotic after sex can prevent the symptoms. A doctor will need to evaluate your girlfriend to determine the best management.