Wearing fajas after a BBL and breast augmentation, could sweating while wearing garments cause infection?

I appreciate your concerns, however after a few days recovery from surgery, your incisions should be sealed adequately to prevent contamination from sweating while wearing your elastic garment.
I counsel my mommy makeover patients to avoid strenuous activity and heavy lifting for several weeks after surgery but do encourage them to walk quietly as early as the day after surgery to prevent blood clots in the legs. If it appears that the weather will be warm early in your postoperative recovery, you can cover the incisions with bandages before putting on your elastic garment. Discuss all concerns regarding your postoperative recovery with your board-certified plastic surgeon. Good luck.

Sweating will not necessarily cause an infection but can cause a heat rash and discomfort. It is best to talk to your PS about ways to make the summer recover more tolerable.