Vaginal bleeding at unexpected times is not normal. It would be important to establish the cause or at least to rule out serious causes. Serious causes can include pregnancy/threatened miscarriage (can happen even if you are on birth control pills), upper vaginal laceration (rare, but can sometimes happen, especially if you are sexually active), and cervical or uterine precancerous lesions. These of course depend on your age and specific risk factors.
Other possible pelvic causes include cervical or endometrial polyps, fibroids, endometritis (infection in tle lining of the uterus). Some women can have spotting while on some birth control pills, but not on others. A simple change of the type of pill can fix the spotting problem. This is something that your care provider should be able to help you with.