I've always had very pronounced/bulging veins on my arms/legs. I've noticed them a bit more pronounced the last day or so. What could cause this?

A number of things. As dr. Namey said, fluid retention or vigorous exercise could affect the appearance of your veins. On the other hand, dehydration can make your veins less prominent. In women, hormonal fluctuations during their monthly cycle can certainly also affect things. As dr. Bein said, fluctuation is normal.
No relation. What you describe could be normal vein patterns...Only with underlying vein incompetence can you determine if you have a problem that needs further investigation/treatment.