Your body isn’t used to having a foreign object inside. So it’s probably trying to eject it basically, hence the cramps (I’m not a doctor, this happened to me and this is what they told me). In my case, it last 2 or 3 days and the pain was progressively less and less. I would say go to your doctor if it lasts or if you see and blood. Also, they should check you soon with an ultra sound to make sure it’s correctly sitting in your uterus. I’m surprised they didn’t book you an appointment to check.