Irregular Period. Haven't stopped?

I have a very irregular period cycle. I consulted my doctor last month when I haven't had my period for 4 months. (I'm not sexually active) My doctor prescribed me Diane-35. She said to take it for 21 days, 1 tablet every day & then I'll have my periods regular. She told me to continue my tablet till 6 months. I had my 1st tablet on 15th Feb. I Finished those 21 tablets on 7th march. & My period started on 10th march. It's the 5th day today. & my period is still as heavy. I am confused about when to start my tablet again? Should I start my tablet immediately 8th day of my missed tablet day? Even when I'm on my period? Or the 8th day from when my period started? And do I still take my pill even when my periods haven't stopped? Regarding my irregular period, I have gained a lot of my weight drastically. (My Vit D & VitB12 levels are too low, so I am on those medications too everyday. Just incase if its related)