I can think of three likely explanations for your observation that your veins are 'shrinking', fortunately none of the three imply a serious medical cause.
The first is that, as we age, our veins do tend to get thinner and more fragile over time. This is just part of normal aging, and it begins to be noticeable probably in middle aged adults. Therefore, depending on how old you are, this might be the explanation.
Another potential explanation is that you have been gaining some weight recently. When we put on extra weight, some of the fat deposits under the skin of our arms and legs where are veins are. This may result in 'covering over' of the veins, leading them to seem like they are shrinking.
Finally, it could be that you used to work out a lot and have recently reduced the intensity of your exercise program, resulting in some loss of muscle tone in the arms or legs. High muscle tone in the arms or legs causes the veins to 'bulge out.'
I recommend talking about this issue with your primary care doctor
to make sure you haven't spotted a serious problem.