I Have Been Found To Have An Overactive Thyroid After

I have been found to have an overactive thyroid after pregnancy. This happened after my first pregnancy and naturally disappeared a few months later but this time round I have been prescribed 20mg Carbimazole. I am on a calorie counting diet but have been gaining weight and experiencing painful bloating. Also sleeping so deeply I have not woken up when my baby is crying. I feel tired all the time. Do you think perhaps I am on the wrong medication?