Different bitches will have different cycles but if one is in heat the other will come in heat also. It is possible she had/has urinary tract infection, the other possibility is a uterine infection,which is scary and an emergency. Is there any odor coming from her vulva? Is she running a temp? If your Vet is not convinced something is wrong but you are have the Vet take x-rays to confirm that nothing is wrong. She could have had a silent heat which basically means that if you had not noticed anything you would not have known she had been in season. Please let us know if your Vet finds anything.