It sounds like SPD (Symphysis pubis disfunction) which can happen post birth, most commonly as a result of a long pushing stage, or pushing with an epidural and legs too far apart. I had it after DS1, and it reoccurred during my subsequent pregnancies. I still suffer now, but not as bad as you.

I also recommend a chiro, but if you prefer I have heard some people having excellent results from a physiotherapist. Whilst you are having this pain try to remember to keep your knees together (for example when getting out of the bed or the car) and consider using a pair of old tights to gently tie your knees together with a cushion in between to stabilise your pelvis at night when you are sleeping.

Also consider doing some core strengthening exercises, which will help strengthen the whole area.

It will get better, but it is important you do not ignore this pain as it may leave you with a weakness which could affect you in the future.

Edited to add: having re-read your post I would like to assure you the size of the baby makes little difference to this sort of pain, it is the position of birth which is the important factor.