My girlfriend was on aygestin (norethindrone) for a month and then stopped, 24 hours later we had sex, is it possible she got pregnant?

Yes: Aygestin (norethindrone) is a mini-pill, meaning it has a single ingredient compared with combination-pills that have 2 ingredients. Mini-pills need to be taken at the same time each day to prevent pregnancy. After the pill is stopped, pregnancy can happen... which is why condoms are wonderful to have around. A home urine pregnancy test is very accurate 3 weeks after sex if a normal menstrual period doesn't come.