Implation bleeding?? Confused

I normally have 5/6 day period medium to heavy, missed a few pills in a row last moth n this month I come on 2 days early and had 2/3 day of light brown bleeding , most unusual! I had cramping after this bleeding have noticed fuller Brests, bloated , eating more and tiered !! I done a hpt but was negative I'm confused as to why I have these symptoms if hpt is negative.. I've read about implation bleeding and maybe it's possible it's to early for the test to pick up that I may be pregnant I don't know help please, I have a little girl already but don't remember this happening Many thanks