If your dad did have something he probably wouldnt share a towel with you. You probably have nothing to worry about. Since you have joined a gym you have been sweating down there a lot more so you could have possibly gotten a fungul infection. I think the best thing you could do is simply go ask your parents, I know it is embarrassing but your dad, or mom, would probably be happy if you came to him/her over it.
Hi its Rob186 again the pimple type thing looks a lot like a boil please HELP