What is going on after my breast reduction? It had a little infection which they cleaned out. (Photos)

Thanks for sharing your pics.  Wound dehiscence  or separation was probably caused by the wound infection.  The deep pink tissue is granulation tissue, a conglomerate of blood vessels, which serves as the "carpet pad", over which the skin will eventually resurface, making the margins join.  Granulation tissue has a "beefy" appearance (from the blood vessels) as well as nerves, which makes dressing changes uncomfortable.  As the wound heals by secondary intention (contracting & resurfacing), your pain will decrease and hopefully. Do work with your PS to agree on a suitable wound care regimen, which allows the granulation tissue to be well vascularized and slightly moist.  

Wound dehiscence, or breakdown, is a potential complication with any surgery.  It's clearly not what any surgeon wants for their patient but is often not something that can be predicted.  If you had another procedure you may have some temporary increase in discomfort but it should be relatively tolerable.  Be sure to keep close follow up with your surgeon.  Wound healing can be lengthy and requires close follow up and patience.
Good luck.