I did a cycling race this past weekend and afterwards jn the afternoon it felt like a vein popped in my calf. what can this be?

Such a pop can-: -be a muscle tear, especially if becomes painful & sore as the day passes, or the next day. A vein does not usually pop during exercise without contact. Both will become sore & both can show bruising as time passes. In either case ice,elevate,& rest initially, and after 2-3 days start heat in place of ice. Slowly start activity as tolerated. The option is to see your PCP- get referrals if needed.