Hi there, I'm so sorry for your loss xxxx
When I miscarried naturally at almost 8weeks gestation, the level of HCG declined pretty slow, it took eight weeks for my cycle to return.
Everyone is different, but only a week after your loss is very early and pretty good chance you still have a substantial amount of HCG in your blood. Because embryos 'live off the yolk sac' for at least seven weeks, your body may still think its pregnant and the decline of hormones in your system will be a lot slower.

Take it easy on yourself xxx I totally understand what it's like to want to be pregnant again straight away, but give your emotional and physical self time to heal. Four to eight weeks in the big scheme of things isn't that much, I know it feels a lifetime now, but let your body heal xox

And best wishes to you having a happy healthy pregnancy in the near future xxx