Late? But negitive?

Olivia

I took 2 tests last week and they both were negative I’ve been cramping the past 3 days but no period? Any insight?my last period and first actual period after my sons birth on October 3rd, was March 15. I bled for 5 1/2 months after having him and got the d shot immediately after my delivery. How long does it take until the d shot wears off?