It's most likely nothing to worry about.
Varicose veins are caused by defective valves within the vessel which results in swelling (you'd know if you had this). Spider veins are similar, but are much less pronounced and are finer in appearance.
Both can be caused by prolonged standing. Find a few pics and see if they look similar.
That being said...how long have you had them? It is entirely possible (and, at your age and with your activity level, likely) to just have a few veins that are darker than the rest. I have a couple small ones just below the ankle on one of my feet.