What r the chances?

Hi jus wondering if anyone could help. I had un-protected sex with my ex bf on the 27th march my last period was 20th march although i had light spotting on the 19th so not sure exactly wot day u could say it actually started. Anyway ive had REALLY bad back ache for last few days and also slight breast tenderness and nausea and feeling tired but this may be cos I had a very late nite on saturday. But back ache is jus like when im due on my period only i dont get it until a few days b4 and im not due until 14/15th april. Any advice would be appreciated.