Have you checked if you have abs separation? If so you should not do any core work like sit ups or planking. Definitely go for postnatal exercises or physio for correct exercises and method to alleviate back pain and flattening belly.
Seeing a chiropractor or a physiotherapist may help as he/she could recommend suitable treatment plan to help ease your back condition. I would recommend getting some professional advice so that you can prevent it from worsening.
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Maybe you can consider doing planking instead? Planking helps strengthen the back and is a wayyyy better exercise for toning of the abs as compared to sit ups.
Have you tried a post-pregnancy wrap or band?
I think you 进风already . That why pain