Can Sex After a Tummy Tuck After 12 Weeks Make Swelling Worse?

Your situation is very normal and will probably resolve with more time, both as it relates to the swelling and the muscle soreness.  It will not cause any long term problems with your surgical result.

Sexual activity is very much "exercise" and that and any vigorous abdominal exercises can still "muster up" some swelling after a tummy tuck.  The reason you are seeing the swelling in the mons (pubic mound area) is that this is the lowest area of the torso (from a gravity standpoint) and can be the area that swells the most. This will get better.  I recommend wearing some compression garments at night if it's not too uncomfortable and also continuing with a good massaging program. 

It is not uncommon for patients to report increased swelling that occurs after vigorous exercise,  even 3 months after tummy tuck surgery. If your swelling is temporally related to sexual activity, I think it is safe to say that it is related.  Similarly, the abdominal muscle soreness is probably related as well.

 Of course, for more precise advice, your plastic surgeon is your best resource.

Best wishes.