What Causes Delayed Periods While On Contraceptive Pills?

hi i think i may be pregnant for the last 4 months...thou ive been taking birth control just for my menstruation cycle and i just missed out one month in these four month and when they came they were fair enough... anyhow ive been paranoid that i might be pregnant..i was on my periods a day back they stopped for a day then came back i keep taking the pills bt my tummy's kinda bloated n have terrible back pains..please help me? can i be pregnant?