Am I just clutching at straws...?

Liz
After my D&C 7 weeks ago I still have had no period (Dr told me to expect it between 4&6wks after). 
Last week I had 3 instances over 2 days of very light spotting and some cramping the day before. I thought AF was on its way.
Nothing yet. Tried testing, bfns. This afternoon I've felt quite nauseous and a little light headed. Is there any chance at all that it was implantation bleeding given the time line? Or would I have had a bfp by now if it was? 
Thanks ladies.