The vein could be inflamed. But because of your numbness, you should see a doctor very soon. It could be an infection or growth pressing on a nerve and a vein.
There is a numb feeling as you said. So i wud suggest to consult a gyno for this. Is there any lump, etc. or a discharge? If yes, it cud be a bit worrying. Many women have protruding veins on their breasts and it is normal, but if you get it suddenly and you also have your arm getting numb, there cud be something with it. If no lumps/discharge,nt a great worry. However, best to consult a gyno.
Hi this sounds to me like a lymph gland problem, but I would get to a doctor ASAP, dont wait about.