What is this post-tummy tuck bulge?

Although there is still healing underway 6 months after a tummy tuck, major contour problems should not exist.  You picture is difficult to interpret.  The abdominal wall may have been poorly repaired or perhaps not addressed at all.  You might have had an untreated seroma that has become organized and hard.  I would press a good board certified surgeon for a specific answer.

An abdominoplasty is performed by excising redundant tissue and tightening the anterior abdominal wall. In some cases, a lesser form is performed whereby only the excess skin is removed without affecting the lax muscles.
In this case, the lower abdomen may still have a lax abdominal wall as a result of minimal rectus plication. This is not always a result of poor technique, but rather, may have been a product of a lesser form of abdominoplasty procedure with minimal tightening of the abdominal muscles.
If you are concerned about the end result, you may consider speaking to a board certified plastic surgeon who will be able to identify the cause of the bulge and repair it adequately through surgery.