Sudden swelling on left cheek after filler

The first thing is don’t panic!  The likelihood of this resulting from a really bad/rare side effect (such as a vascular occlusion) 11 days after is unlikely. Infection is also unlikely at this stage.  Many things need to be taken into account when assessing a possible adverse reaction post filler.  How much volume was put in; was it injected with a needle or a cannula; is there any discoloration of the skin overlying the swollen side; is it unilateral or bilateral; are you experiencing pain among others.  The fact that it is not very noticeable and that it is now 11 days since the procedure is a good sign.  Compression from sleeping on that side or water retention overnight could be the culprits.  Hyaluronic acid based fillers draw water in so swelling is not uncommon.  Swelling can compress nerves which may lead to the eye twitching.  Side effects from HA fillers usually resolve by week 2 although they can last longer – even as long as 4 weeks. Taking a blood thinner or anti-inflammatory (aspirin or advil) or using an ice pack should help resolve the symptoms.  I would certainly call the doctor who did the filler and follow up with them. Nothing beats an in person assessment with the doctor who did the injection as they know exactly how much and where the product was placed.

Typically this does not happen 11 days out.  If it is happening on just 1 side, you will have to rule out infection.  Infections normally are red, swollen, tender, and warm and are accompanied by fever but they don't have to be.  You will need immediate medical evaluation to rule this out.  Once this is ruled out, you will also have to consider some form of allergy/rejection to the product.