Lump after Tummy Tuck?
Most likely this represents something called fat necrosis. 7 weeks post-op is still quite early and often something like this will resolve itself over time, but that may be up to 6 months or longer. If it persists then revision may be indicated.
Concerns regarding your results
are certainly understandable when everyone desires perfection... but if your firmness is at your scar, you can assume its scar or fat necrosis and this should slowly get smaller as you continue ti heal. It may not go away completely and if you have residual bumps and contour irregularities, you should ask your surgeon about what could be done about. You should find out what the revision policy is as well as you may need it.
What is this lump and will it ruin my results?
At 7 weeks after surgery, the lump, if quite hard, is usually caused by fat necrosis, death of the fat along the incision. It often softens over 8-12 months, but if not and if painful it will need to be revised.