Thank you for your question. It is not uncommon to experience pain after muscle repair and abdominoplasty what attempting vigorous physical activity like a sit up.
That said however if you're my patient I would want you to see me for an examination and if he were experiencing pain this far out from abdominoplasty.
I have my patients wait a full three months before doing any exercise that directly stresses the abdominal muscles. Even then, whenever you add an activity back into your exercise regimen I tell patients to take it slowly and "listen to their body". If you overdo it, your body will let you know with pain or discomfort. If that happens, just back off and take it slowly.
Hard to say but I bet a few more months are needed before you feel better. You may injure or tear the repair work if you do too much too soon. Go easy!