Implantation bleeding?!

I was due on 16/5/16 had slight pink/brown on pad 18th/19th then today started with light bleeding. Had the feeling I had before Christmas when we fell pregnant as all stomach had an aching feeling all week. Took a test on Thursday but negative result. Just wondered if this can be implantation bleeding or probably just period. Never usually late but with having a negative response got a little disappointed. Any ideas?