I Had A Uterine Septum Resection 2 Months Ago And

I had a uterine septum resection 2 months ago and have had normal cycles since. However this month, a few days before my period I passed a strange black jellylike mass and had some spotting then my period came later then usual but seemed pretty normal. Since then the dark discharge has returned along with nausea, abdominal pain and lightheaded feeling.
I went to the GP and he said it was probably just my body healing from my surgery but the nausea and dizziness is getting worse and it just doesn't seem right.