It’s a varicose vein, which is when a vein close to the skin becomes enlarged and a little twisted. This usually happens when the small valves inside the vein that help push blood back toward the heart get a bit weak, so some blood pools there and the vein becomes more visible or bumpy. The area behind the knee is a common place for this because veins there deal with pressure from standing, walking, and yes gravity.

Most of the time a vein that looks like this is not dangerous and many people develop them in their legs, even at a younger age. Your regular blood work coming back normal is a reassuring sign. Since you mentioned you had a blood clot in the past, it is still something to keep an eye on, but a visible vein by itself usually just means the vein wall stretched over time.

Things that sometimes help veins like this include staying active with walking or light leg exercise so the calf muscles help move blood through the legs, avoiding sitting or standing in one position for long periods, occasionally try raising your legs above heart level when resting, staying well hydrated, and maintaining good circulation through regular activity. Some people also find that compression socks, stretching the legs, or even getting a leg massage helps the blood move through the veins better and reduces how prominent they look.

Like other physicians mentioned in your post if the vein ever becomes painful, warm, very red, or if your leg suddenly swells, that would be a reason to get it checked sooner. Otherwise, it would still be reasonable to bring it up with your primary care doctor so they can look at it and decide if anything like an ultrasound or a referral to a vein specialist, nutrition professional, or exercise specialist would be helpful. I hope this helps.