What Causes Irregular Vaginal Bleeding?

hi mam...i had periods on last month 25th of nov for 5 days discharge.but this month i had 16th to 19th whitish and grey mix discharge and 20th to 22nd brick and brownish/black discharge and 22nd night i had red coloured discharge(like periods) but i had discharge only for one day with heavy pain other remaining 4 days very minimal discharge(too less) compare to my earlier periods...please let me know the reason for this discharge ASAP....i am bit worried