My guess is that your bird is actually an Easter Egger rather than an Ameraucana. Many feed stores, hatcheries and others, advertise that they have "Ameraucanas" when, in fact, they are actually Easter Eggers. The only way to know for sure that you have an Ameraucana is to get one from a reputable breeder who will also specify what variety (color) of Ameraucana they have. Plus, Ameraucanas should have slate colored legs, whereas yours looks to have a yellow/willow colored base. Either way, they are both great birds that lay great eggs.
That said, 5 weeks is a little young to start sexing birds. I am only seeing one row of peas on the comb, which would indicate pullet. However, what concerns me is that the comb seems a bit pink for only 5 weeks, which would indicate cockerel. If you are right on age and if the color on my computer screen is accurate, I think I would have to lean toward cockerel.