Breastfeeding and 14 days late.

I'm breastfeeding my 9 month old and my period returned at 4 months postpartum and has been on time ever since. But now I'm 14 days late and have taken 4 pregnancy test and they have came up negative. I also am feeling nauseous all the time and extremely tired. Is it possible that I'm still pregnant or could it be I'm just skipping this month? I have a Doctors appointment Friday to find out what wrong just could use some opinions before I go.