I usually used to have normal blood flow, but from past few months I have started getting very low blood flow (mostly black blood) for starting few days and then the normal blood flow starts. This month also it started like the same, but after 2-3 days it suddenly stopped. Few months back I had done all the test including pelvic scan and everything came normal apart from Vit D deficiency for which I took the tablets prescribed by doc and the course is finished long back. Do I need to do any additional tests? Is this kind of change in menustral cycle normal or not?