HPV and scratching.?

They are over 100 HPV types 40 are linked to the genital area. The virus is site specific meaning the HPV types that affect the genital area do not cause warts at other non genital area.

The nearly 40 types of HPV that have been found in genital warts[16] are highly host-specific. HPV types demonstrate a high degree of site, with some types only found on certain parts of the skin or mucous membranes. As a rule, HPV types causing common warts of the skin do not infect moist epithelium, and vice versa.

http://emedicine.medscape.com/article/219110-overv...

Not a risk to your back...a risk if she touched your genital area or buttocks

HPV is a virus. The potential for infection can come from any body part swapping body fluids.

Were you bleeding? Was she? Your risk is pretty low, but HPV can be contracted from elsewhere besides the crotch.

Prolly

nice knowing ya