2 Missed Periods??

Kayla
I just gave birth almost 4 months ago. I had a normal period in June but then I missed my second one despite having all the normal PMS symptoms. Now I'm several days late again. I don't breastfeed at all because I work so much. I've taken multiple pregnancy tests that all came back negative too so I have no idea why I'm missing them. Should I go to the doctor?