Implantation Bleeding or Period?

Hubby and I are TTC since last month. I missed last month's period (was due 16 days ago). I thought for sure I was pregnant but I guess not (HTP and blood negative). We have been still trying and today I am experiencing light cramping in lower abdomen (like a period) and light spotting (one panty liner all day - was reddish/pink and thin at first, now brown). Could it be implantation bleeding? I was under the impression I wasn't due to ovulate yet since my periods usually start on the 27th of each month. Or could it be my late period? I'm so confused!