What you are describing is typical for varicose veins.
When you stand up blood flows into them and they dilate. If you are resting on the couch with the legs elevated blood flows out of them and they disappear.
Any increase in body temperature as in showering or exercize will cause venodilation - cause an increase in the size of the vein with more blood pooling. Venous insufficiency can also contribute and you can and should get a venous Doppler ultrasound (Duplex scan) to see if you have this condition. Once this is treated endovenously (EVLT Never Touch, VNUS Closure or ClariVein), most of these will reduce in size and abundance and thereafter, you can get additional treatments such as foam sclerotherapy to obliterate these veins (spider veins, reticular veins and varicose veins). Read the E-Book about venous insufficiency on the link below - it has information, pictures and diagrams.
It sounds like a varicose vein, which could be caused by venous insufficiency, which is caused mostly by genetics. You need to have an evaluation and an ultrasound to determine the cause. If you have pain, swelling, and even restless legs, treatment can not only eliminate the varicose vein, but improve your quality of life!
Varicose veins typically enlarge when you stand and decompress when you lie down. The usual reason for the varicose vein formation is a malfunctioning (refluxing) of the valves in the saphenous system of veins. What you describe sounds like varicose veins. You should see a vein specialist for a full venous evaluation including a venous reflux ultrasound.