Most protein shakes
contain nitrates which can cause vasodilation
of your blood vessels and causes decreased blood pressure and thus causing dizziness when blood flow decreases. High protein intake can cause proteinuria
and can cause damage to kidneys. Do not take protein shake immediately after doing the cycling have some rest around 1 or 2 hours and then take it, or else you can try taking carbohydrate
shakes. Mainly for the repair of your muscles, rest is more needed than the protein shake because you do not substitute your meal with the protein shakes.
Have a good Health.