I've had 0.5ml of Juvederm lip fillers and I'm breastfeeding! Should I stop breastfeeding now?

Lip fillers are done locally and there is no systemic absorption so breast feeding should be safe but again no studies done. 

If you already got your Juvederm lip injection when breastfeeding, it will not affect the baby and you can stop worrying. I would never inject Botox or Dysport in a person breastfeeding though. If it is possible, avoiding injectables is recommended until the breastfeeding is stop.

The fillers should not affect your breastmilk, but as you mentioned there are no studies available to review.  You can discuss it further with your doctor just to be sure.

It's my opinion that a small volume of a local injection of hyaluronic acid with lidocaine (most versions of fillers now come mixed with lidocaine) should pose virtually no risk of significant systemic absorption and reincorporation into breast milk in enough concentration to hurt a baby.  As a medicolegal and common sense precaution, most providers in the USA do not inject cosmetics into pregnant and breastfeeding patients.  While you could pump and dump to play it extra safe, I would not recommend that for more than a day in this scenario.