Large hard lump under skin after TT: Is this scar tissue? Or underlying infection? (photo)

Hello! Thank you for your question!  This lump that you state is likely either a small pocket of seroma or some scarring/fat necrosis at that area. On your photo, I would favor the latter as I do not appreciate any significant fluid collection. If it is the latter, it will likely resolve over time. Massages no of the area will help. If it does not resolve or a wound develops, surgical excision may be needed. Regardless, I would follow up with your surgeon for an in-person evaluation and recommendations for care to the area. 
Hope that this helps!  Best wishes for a wonderful result!

It is not unusual to have irregularities around the scar postop following a TT. Best to run this past your PS but most of these will resolve with massage and scar management.

You haven't mentioned how long ago your procedure was.  Due to this change in your condition it is advisable to contact your surgeon for an appointment.  An examination should help determine the nature of the lump you describe, as well as any interventions that may be necessary.