implantation

Morgan
Hello! I have a question, I am confused! 
So I have PSO syndrome a very mild case.I am using an application for my ovulation, and keeping record of my periods as having PSO I used to have very irregular periods. Since coming off contraception 4 months ago I have finally become regular usually if late only 2-3 days. I do have regular unprotected sex with insemination. I did have constant sex during my fertile and ovulating period that the calculator estimates. Today the 19th of December, i had no bleeding early morning and when I got to work at around 5:45 am I felt a big wet amount in my underwear, I then discovered that I have a very heavy, clearish and bright red blood in my underwear. I didn't have any light starting to this (period if that's what it is) just straight away heavy and lots of it. I am due for my period roughly around the 23-24 of this month as that is when I got it last month. I have NEVER been early before, I have sore back and cramps in my stomach, loss of appetite and just feel off, and over the past could of days having light headaches every morning. I have had aching legs the past week or so as well  is implantation bleeding ever heavy, or am I early for once on my men's trail cycle. I have read and researched it is posssible to get period type blood during pregnancy. I am 10 days past ovulation which is around the right date for implantation bleeding. I am hoping I may be pregnant. Is this possible if any one has any similar experiences please let me know. Or is it just my period.. it's just weird it is early, no light bleeding, extremely heavy like soaked straight through underwear, sore lower back, headaches. 
Update: I have now had no big gushes and it is just a light colour and quite watery.. I have slight cramps in my upset vagina lower stomach area, I don't feel like eating. Thanks so much