period came 8 days early?

my period came 8 days early .....never happened before - im not on the pill . had sex aweek after my last period.

I had very sore breasts for 5 full says before i 'came on'.....the day i did come on i saw a big bloody blob in the toilet -

i had sex the night before i came on  - could this have brought this on? did a test was negative

can anyone tell me whether this could have been a possibly pregnancy? was the bloody big blob a clot?  

i never have really sore boobs like that even when im due on! they hurt - the day i came on i felt sick and had  a headache - anyone had similar situation?