Polymenorrhea or bleeding from vagina more than once a month could be due to several causes. However in your case the most likely cause is the BC pills you are taking, which are most probably not suiting you. Other causes include fibroids, adenomyosis, endometriosis, uterine polyps, hormonal imbalance, thyroid problems and bleeding disorders. It can also be a part of severe pelvic inflammatory disorder. STDs like Chlamydia and gonorrhea too can cause this.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your gynecologist for this. Take care!