Miscarriage/D&C

I was pregnant by using pre-seed and I was about 12 weeks pregnant. My baby didn’t have a heartbeat for 2 weeks and quit growing 3 weeks ago. I found out at my next appointment,no cramping or even signs of a miscarriage so I had a D&C; done. How soon after a D&C; and miscarriage taking pre-seed can you get pregnant? And how will you know because I still test positive on pregnancy tests with my hormones. (Note:no I haven’t had sex yet with my husband,it’s only been 3 days since the D&C;)