My foot veins usually pop out but recently they have been flat. anything to worry about?

No need to worry: if you are otherwise feeling well. Flattening of veins is a common sign of dehydration, so you may want to try drinking more fluids to keep yourself well hydrated. Veins on feet tend to be more visible "pop out" after standing up for awhile, due to gravity resulting in more fluid remaining in the feet and legs. Veins in the feet tend to be flatter in the mornings after laying down all night

Foot veins.: Foot veins tend to enlarge when you are standing and decrease in size when recumbent. This is due to the fact that the pressure in the veins in the legs is increased when the veins are below the heart and reduced when the veins are at heart level or above. What you describe is normal.