skin under left eye rapidly thinning

Around January this year I noticed that the skin under my left eye was starting to become a bit veiny? I was a bit worried because it wasn't there a month ago but I ignored it. Fast forward to April now and it's become a bit more veiny which I think indicates that my under-eye skin is becoming thinner and thinner. But I don't know why because it's only on the left under-eye and this is happening in only a span of a few months. When I gently touch my left under-eye it feels a bit..stingy?I feel a slight bit of pain.
Actually, I think in these past few months, I've been losing a lot of fat around my jaw and it's not because of weight loss. I'm so worried about why this is happening and if this will continue to get worse.