Hi Tegan,
It's normal for stretch marks to get itchy. I am not sure what the "medical" reason is for it but, as someone that is prone to stretch marks, mine get itchy when they are fresh. I started getting them when I hit puberty. My skin grows, I get stretch marks. Being overweight doesn't help but I can't do anything about them so I have learnt to accept them. They fade anyway.
The itching will also go away as they become a little older. Perhaps there is some dermatology cream or something that you could apply to stop the itchiness? Best to ask the Dr. or chemist I guess.
Hope you don't get too many more!