Period or implantation

So according to my app I start my period in two or three days. I am also 19w postpartum. Yesterday and the day before, only in the evening, I’ve had light pink stringy discharge. Sorry TMI! The night before last I had really bad cramping on my left side. If I start this will only be period number three after having our baby#2. I started my period way earlier this time after birth. With my son I didn’t have one for over a year.

If we think it’s implantation bleeding when should I test?

I didn’t do all this looking into with my son and daughter. So I’m not sure what to think. Haha!