What Causes Inter Menstrual Bleeding?

i have a question i always start my menstural cycle on the 4th of every month give or take a day or so and they last about 5 days this month i started on the 14th and on the 17th i stopped then on the night of the 18th i started again with clumps i never clump then the 19th there was nothing and then i just started again i have never done this except 4 years ago after giving child birth i need to know is this normal or what could be happening to me?