When can my gf take a pregnancy test?

my gf is freaking out she is only 3 days late (she normaly starts exactly on time every month, like clockwork), and thinks she might be pregnant but she recently had a miscarriage in march and shes not ready to have another baby as shes to upset she will lose this next one and she wont be able to handle it. she has taken 2tests one at night one in the morning both neg. she had what appered to be implantiontion bleeding 11 days ago and we�ve had unprotected this month also. should the test show positive yet or can it take a bit longer? also if you�ve had a miscarriage before are your chances the same next pregnancy or is the risk higher (doctors did explain but we were to busy thinking about the baby so we�ve forgotten) thanks for your help