Hives after roller derby practice?

I play roller derby twice a week. Every practice, when I gear down, I have hives where my knee and elbow pads are. Not my wrist guards or helmet or skates. I have moderate eczema, but it is under control. I sleep with a humidifier, and I drink lots of water. I also use lotion 2x a day. What could be causing this, and what can I do? I'm sick of itching like crazy after practice!