What is best practice to treat abdominal bulge and hernia after DIEP flap?

Bulges after DIEP flap harvest are a possibility (as well as free TRAM) - and this can be weakness in the abdominal wall, a true hernia, or possibly fluid. 
Revisit with your surgeon for evaluation and likely further tests would be needed ie. CAT scan. 
In the mean time - if no pain when it bulges or any other symptoms other than cosmetic - you can wear an abdominal binder for some support - likely this can be fixed after the precise diagnosis is made. 
Best of luck.

I'm sorry to hear that you are having trouble after your reconstruction. Bulges and hernias are more common with TRAM flaps, but can occur with DIEP flaps as well. You are probably aware that during a TRAM flap, the rectus muscle is taken along with the skin and fat for the reconstruction and weakens the abdominal wall. DIEP flaps preserve the rectus muscle, but if the nerve supply to the muscle is interrupted the muscle looses its tone and will result in a bulge. Simply reinforcing the abdomen with mesh will repair the hernia, but will not completely restore the muscle tone in the abdominal wall. The repair will require recruitment of your oblique muscles to restore abdominal tone and is successful in correcting the issue. I have treated many patients with both TRAM and DIEP flap bulges and would be happy to walk you through the procedure, explain the anatomy further and why it's successful. I hope this helps.