This question is difficult to answer, because I don't know the laser that was used to treat your spider veins. I'm going to speak generally on the lasers that we use here at the practice to treat spider veins. To clarify, our practice recognizes spider veins to be less than two millimeters in diameter. Spider veins that are less than two millimeters in diameter are more responsive to laser therapy and sclerotherapy, as opposed to veins larger than two millimeters, which often require mechanical stripping by a vascular surgeon. Typically, we'll use the pulsed dye or VBeam laser for very fine telangiectasias or spider veins.
The bruising that you're experiencing is commonly seen with this therapy. In fact, it's a good sign that many of your small spider veins are responding positively to this treatment. The bruising that you're seeing is red blood cells that have been spurt out of the small blood vessels, and the result is the discoloration that you see today. This usually takes two weeks to resolve, and after it resolves, there's no small spider vein underneath it.