New Portable sauna-New Sores on my Back!!

I have been using my sauna for about a week, and with in the 1st few days developed sores on my back only, and they kinda itch, any thoughts on this, why only on my back, I know I am sweating but thats all over, Also is it better to go in with less clothes or in a t-shirt and sweat pants for the sweat to soak into? Thanks for any info~