One reason why you may not be able to lose weight is as a result of a lack of sleep. When you don't get enough sleep, your body experiences physiological stress and, biochemically, you store fat more efficiently. I would advise that you look into getting a sleep trainer for your little one. This may be beneficial for you, and your baby.
Women vary in how quickly they lose the weight gained during pregnancy. Go slowly and try not to be overly concerned at this time. With proper diet and exercise you will stay healthy, and find your weight does begin to return to your pre-pregnancy level.
It is always advisable to visit your medical practitioner for a general examination to assess your overall state of health. They will be able to tell you what other blood test you may do to rule out any pathological causes
All the best