I should tell you right off that I've already answered one question about this, and I am one of the happy Mirena users, after nearly 5 tears of having it in.
One thing I wish I could have told you was to wait at least 6 months after your baby was born to get it put in because your body has already been through so much, and then to introduce this to the mix can be too much for your body. Not only is there a very low dose of a synthetic progesterone - even a very low dose is SOMEthing, and we all respond differently as we know! - but the device works by irritating the uterine lining in such a way that it sloughs off and becomes thinner. In a body at its normal state this would happen without even being noticed (our bodies do it every month), and this is also why many women stop bleeding during their periods when they have the Mirena device in (I did). But you are probably still recovering and very in tune with every change and, like you said, twinge :) that is happening.
If you can talk to your doctor about it, that's what I would recommend, but if not, you can always have it removed now and try it again if you choose to later when your body has recovered more. The only thing stopping you would be a huge co-pay, but in the end, your body and your peace of mind are far more important. Best wishes and good health in whatever you decide!