Bladder issues, of many different kinds, go hand in hand with menopause I'm afraid. As does vaginal atrophy. The vagifem has been prescribed because it will give a dose of oestrogen to the vaginal tissues and help to plump them up a bit. It will do the same for the bladder because it's so close.
It is normal for menopausal women to be back and forth to the GP with what they think are bladder infections. Sometimes they are, because the weakened bladder is prone to them, but half of the time there is nothing wrong. It feels wrong because it's weak.
You need to do some pilates exercises to strengthen your pelvic floor and keep on with the vagifem.
And no, it's not cancer.