Hi welcome to Health care magic forum.
Thanks for calling H.C.M.F.
As you describe your period started yesterday, and ther was pain, by the time you wake up today there was heavy bleeding, as it has happened for the first time it could be due to infection, which caused the pain and followed by menorrhagia
The other cause could be anaemia
, So i advise you to consult a gynaecologist for diagnosis and treatment.
Wishing for a quick and permanent recovery .