I just had Lutronic Infini treatment done 3 days ago and now have a saggy double chin. Is this a temporary side effect?

I have not known Infini to cause sagging.  In my practice, I utilize Infini to tighten and define the jaw line.  What you may be experiencing is the swelling that is expected after Infini treatments.  In general, the swelling subsides in 3-4 days.  To aid resolution, try applying ice to the area, but never directly to your skin.  It would be unusual for any swelling to persist indefinitely. Consult your treating physician for further guidance.

This swelling is expected and will resolve. There should be no worries. It takes about 7 days to resolve. 

This is definitely due to swelling and is a temporary side effect. Within 7-10 days this will improve. As with all treatments that rely on collagen stimulation, the best results are seen 2-4 months after your last treatment. INFINI RF is one of the better micro needling devices as it uses insulated needles to deliver RF, and combining this with PRP can give a synergistic result. All the best, Dr Davin Lim. Brisbane. Australia.