Popping a stitch: My diastasis was severe. 10 days post-op.

Congratulations on your surgery! The discomfort you felt after sneezing is expected.  It is unlikely that you did any damage to your surgical results.  You may want to consider allergy medication to help your symptoms however for your comfort.  Best wishes.

Congratulations on going to what sounds like an excellent plastic surgeon who did your rectus diastasis repair with three layers of plication.  I am sure he or she did this because of your wide separation of abdominal muscles.  However, this triple closure of yout muscle separation makes it very unlikely to pop a stitch based on coughs/sneezes secondary to allergies.Since your personal plastic surgeon knows better how he or she repaired your diastasis, it would be appropriate at this time to make a phone call to your plastic surgeon to get the best answer to your very reasonable question.  

Hi, Chemmommy.  Popping a stitch or having a stitch pull through are always an early concern after a diastasis repair.  However, a multi-layered repair helps to reduce the likelihood of a broken stitch, or that if a stitch does fail, the other sutures in the repair may compensate.  
If you have developed a focal bulging, then there is a high likelihood that a stitch has broken or torn through.  If your abdomen essentially has the same level of flatness that it did previously, then it is more likely that you tugged on the muscle with your sneeze.  However, there could be some localized tearing of the muscle at the site of the suture that got stressed -- I caution my patients to be extra protective with their muscle repair after such an event, to be sure to wear their garment for additional support, and to use a small pillow to hold the abdomen during coughing/sneezing/laughing.
There is also a possibility of the diastasis repair unraveling or loosening if there has been a suture failure.  This would cause the abdomen to begin to protrude again.  However, more common for some loss of the immediate post-operative flatness, is inevitable relaxation of the muscles due to pre-existing weakness as a result of a long-standing diastasis.  Once the muscle repair is healed and very solid, and your PS has authorized you to begin toning, the flatness will begin to return.
Your PS will be the best person to determine whether some concerning damage occurred with your sneeze event or whether, and more likely, you just pulled a little on the muscle.
Best of luck, and I hope you continue to heal well and have amazing results!

At 11 days post op you could still pop a stitch.  Wearing a tight compression garment will give you added support and protection.  Follow up with your surgeon on a routine basis.