Yikes

My period was due 5 days ago 
My nipples are super sensitive 
I had brown spotting for two long days on what should have been day two of my period 
My period is extremely irregular (I had lost it completely due to eating disorder for about a year. Just came back 4 months ago) 
I had unprotected sex 4 weeks ago (a few days before ovulation) 
Spotting happened 3 weeks after sex (is this too late for it to be implantation bleeding?)