Can any doc explain why are my veins so prominent all of a sudden?

Location matters: Location of the prominent veins matters. If it is all over(arms/hands/legs, etc), it may be related to increased fluid load (i.e. after having a high salt meal), or due to ambient high temperatures. Veins dilate(become prominent/bulge) to help regulate body temperature (cold makes them 'shrink'). If it is just in one body part(a leg/arm), they should be evaluated to make sure nothing else is wrong