Are lumps common 12 weeks post tummy tuck?

Young,
These lumps are most likely the knots of the sutures used to tighten your abdominal muscles, assuming that you are very thin. If you are not very thin, then they may be small nodules of scar tissue that form in areas of fat that are no longer alive. See your surgeon for a more definitive answer. Good luck!

Thanks for your question -
These lumps could be several things -
They could be hematomas or seromas (fluid collections below the skin).
The could be fat necrosis (small hard lumps of fat that hasn't survived).
It could represent sutures or part of the muscle plication.
It could even (much less likely given the presentation and location) represent a hernia.
The only way to be sure is to see your plastic surgeon and have a conversation and examination with him or her.
Also important parts of the story would include when these issues first came up, are they getting better, etc.
It may be something that will resolve on its own over time.
I hope this helps.

As stated in the other answers, there are many possible causes for these lumps.
In any case you should see your surgeon for the explanation but it does not sound a problem that should cause you any immediate concern.
Given that you are only about 12 weeks after surgery, your muscles and scar tissue are still acitve and may explain the tighness you describe after sitting for 5 minutes. I have found that performing core strengthening exercises as advocated by Pilates and Yoga instructors is beneficial in this regard.
However, I would advise that you seek your surgeon' s permission before undertaking an exercise regimen.