Dear Jade, I'm so sorry for what you've been through, with the miscarriage and now this diagnosis.
It is possible to get gestational diabetes more or less from conception. I am a member of a fb group GDUK and some ladies have had it from 1, 4, 5 weeks... These are ladies whose pre-pregnancy blood tests were diabetes free. It seems to be a condition where there a lot of unknowns. So more tests are *definitely* in order.
Stay away from the insulin and enjoy your wee one. They grow up so fast! I will keep you in my prayers. Be well,
Hannah