I had a filler injected in cheek area, Restylane. Can that cause eyelid swelling and under eye hardness 3-4 months later?

Please go back to your injector and ask them to dissolve the product. That could help the situation. Since the Hyaluronic acid (Restylane) is a hydrophilic product, any inflammation near the area of the fillers can cause swelling of the injected area. This might not be related to you reacting to the product but having inflammation from other causes. The best is to dissolve the product and see what happens. I also, would suggest to try a course of Medrol pack if there is no other medical issue or contraindication.  

Fillers can cause some swelling. However, if you recently had a bug bite and it has been several months since your injections, it is lily more related to the bug bite. Best to be evaluated by your doctor.

Hi, it does sound like this is a reaction, or problem, due to the filler.  I've never heard of a gnat bite, but it's possible to get a small infection of the filler product, perhaps if there was a bite, which can lead to inflammation, hardness, and a "granuloma" reaction.  This isn't you reacting to the product really, as much as it is likely what I think is a commonly misunderstood concept of how granulomas form when fillers are present, and that is from a chronic, indolent infection.  I would recommend going back to see the injector first, and then someone who is a professional that does fillers.  Hyaluronidase may be a good option to get rid of the issue faster.