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I believe this is the first episode of heavy intermenstrual bleeding or polymenorrhagia for you and during the mid cycle.
The most likely cause for the same can be irregular shedding which can occur in case of ER estrogen excess or progesterone deficiency where endometrial proliferation is unopposed by deficient in progesterone. The fact that you are on birth control should not be presenting in this manner. But, in cases of high dose oral contraceptive pills with higher estrogen content such as irregular shedding has been commonly seen. I would like you to let me know the name of the birth control which you are presently using and since how many months? Secondly, as there was ample mucous discharge mixed with blood on per vaginal examination it is better to rule out any local vaginal infection but invariably it will not cause uterine bleeding or endometrial shedding.
So, the possibility can be either an endometrial polyp or submucosal fibroid which may have developed over the last few months and are likely to be the cause or there is progesterone imbalance which is a likely cause. Hence, ideally antifibrinolytics should have been advised to control and reduce the bleeding and follow it up with an ultrasound pelvis to look for any polyps, submucosal fibroid or ovarian cysts indicating endometriotic cause. As the pain is diffuse in the lower abdomen and per vaginal examination was painful on deeper examination, endometriosis or PID (pelvic inflammatory disease) should also be ruled out by USG and swab test. You should start yourself on antibiotics empirical treatment which shall take care of PID or any bacterial vaginosis as well.
If the bleeding is not controlled and remains heavy causing anemia and dizziness as manifestation then to stop the bleed the best way would be to undertake a dilatation and curettage procedure. Through which can remove the endometrium and subject sample for histopathological exam to rule out any endometrial hyperplasia and accompanied atypia, which may indicate towards an endometrial cancer depending on the reports. At present, wait and watch for excessive bleeding for few more days and if it does not reduce then review with ER and get the ultrasound done sooner and further evaluation. Keep yourself well hydrated to maintain the blood volume and avoid dizziness episodes.