Why does this happen what i weight train?

My problem is with only my arms. I am doing p90x and have been doing it for about 7 months. I notice a big difference but whenever i lift weights my arms go numb and become swollen. They don't hurt at all but im bothered by the swelling and numbness. What is this? I was told its caused by a collection of blood but im not sure. If im training to failure...shouldn't it be a burning feeling?

Thank you :) im happy its nothing 2 worry about.