I agree with your ob and mango. Most of it would probably be caused because the weight and different kind of balancing act for your body now... I think the key is to focus on the core (lower abdominal) muscle groups, which all the above recommendations should target. Other thing you can try is core work on a fitness ball, which can also be easier on your back (I have back and neck injuries). Another thing that can help is Pilates, but you really need to be careful to learn the very basic core support of pilates.
Definitely something I'd do with supervision first few times, no matter which method... I have been warned they all have potential to harm if you don't know what you should AVOID doing.
I think swimming uses every muscle group though. It's relaxing, and good if you are carrying a lot of weight like me. You can even try some hydro excercise classes. I can't remember what they are called but they help stretch and use your muscles, and you don't feel you've done the work until you come home
Alternatively, a relaxing bath may help short term.
Take care