I realize this topic is old, but for those that come across it, should know this.  It is probably razor burn.  DO NOT use someone elses razor to shave, especially your private area, guaranteed razor burn.  It has to do with the transferring of body oils and what nots. 
Do not use another person's razor, shave in the direction of the hair growth, use soap or shaving cream to lubricate, and use lotion to soothe when finished.  (PS, I always use a new razor when shaving my private area.  Never had a problem with razor burn.)