Hard Bulge After Tummy Tuck with Muscle Repair

This is likely a combination of scar tissue and compromised fat around the belly button which is poorly vascularized.
Generally over time, this will resolve.
The tight feeling also tends to resolve unless you have gained weight since undergoing the surgery.
As mentioned, discuss your concerns with your surgeon to ensure that all is healing well and there is no other cause for your findings.

Hi there-
It is, of course impossible for me to know for sure without examining you and having a better understanding of how your surgery was done (there are many ways to tuck a tummy), but is sounds like you are experiencing some contraction of the scar tissue that formed under your skin.
Just as scar forms in the skin, it also does so underneath at the site of your muscular repair. All scar tissue is very inflexible, and also tends to shrink (or contract) over the first year or so. With certain movements, this will cause most patients to feel exactly what you are experiencing.
It is important to visit your surgeon to be sure you don't have a more significant problem (doubtful from the sound of things). It is also important for you to understand that at this point your wounds are about as strong as they're going to get- in other words, it's unlikely that these feelings mean you're causing yourself harm. Once my patients reach complete healing, I actually encourage them to do stretching exercises and yoga, to stretch the scar slowly over time and prevent these uncomfortable feelings.
Visit your surgeon- as long as things are good on exam, try stretching gently once a day.

Tighteness around the umbilical region after a tummy tuck is not uncommon. The firmness usually softens over time. This may take up to a year.