Hard Lump After Tummy Tuck?

This definitely is where you need better communication between you and your plastic surgeon.  Please ask him or her exactly what they believe this lump may be. 

Having said that, this is very unlikely or almost impossible to be an incision hernia since there are no incisions below your abdominoplasty incision during a tummy tuck.  My best guess is that this is probably some blood mixed with fat in the area of your mons pubis, or when placing the deep sutures to sew together the SFS (superficial fascia system), your surgeon may have caught a large bunch of tissue here, and this should vanish with time. 

In my practice, I would use ultrasound on this area (the kind the physical therapist use for sore muscles and not the ultrasound used to see the baby in your stomach).  This brings blood supply to the area and removes most lumps. 

If this lump doesn't go away by 3 months after surgery and you still don't feel comfortable with what your plastic surgeon told you, then a second opinion of another Board Certified Plastic Surgeon might be appropriate, but I am pretty sure this lump will vanish by then.

This is most likely secondary to a small hematoma.  Discuss with your surgeon.  It should improve over time.