Depo period?

I am now 40 days late for my period. I took the depo shot for the first time in early June and was due for it by September 7th (the latest) but me and my husband want to start trying for a baby so I didn’t go to my appointment. (I only had 1 shot). I got my period every month but it was about 19 days late in October but it came and ended normally. Now I’m 40 days late but pregnancy tests are coming out negative. Anyone have similar experiences? What should I do? What Can I do? Any advice? And could I possibly be pregnant? So lost and frustrated!