Sometimes it depends on the kind of girth you are using and what materials it is made out of. My mare started to get some serious galls until I realized that the girth was actually pulling her hair out...by the skin (luckily it only happened once). She never did "tell" me she was being hurt. When I went for the girth, I realized that the underside of it that was against her belly was more like a tacky-tack material than nylon...tacky-tack is NASTY for a girth. I quit using it then and there. Now I use a neoprene girth, but you also have to be careful what kind of neoprene you use. There are some out there that have the really thick padding...that's not good because it soaks up the sweat and grime eventually and you just can't wash it off, which means there's lots of bacteria growing in there that could make sores. It also starts to fall apart, which I've found rubs terribly and irritates their skin.
My girth that I had trouble with was a western girth, and I replaced it with a Big Horn Tuffy. This is the thinner type of neoprene, and it seems to be a little more compact and firm than the big spongy girths. It has yet to fall apart. It still looks brand new, even, and I've never had another problem.
However, I will say that it could also be that your girth is too tight. When I first started using my Tuffy, I felt like I needed to keep tightening it, but what the problem really was was that the neoprene was helping the girth behave EXACTLY as it should...it's not really to hold the saddle on all by itself, just to help it be more stable. It was a blessing in disguise because I just figured, "Well, I guess I'll just have to improve my balance."...and I did. You should actually be able to slip 2 fingers comfortably between your girth and the horse. If it's too tight, the skin can't breathe.
I don't know if they still make them, but I also have an english girth that's nylon string. It's kinda like the western cotton string roping girths, but it's for english and each string is braided nylon. My horse really likes that one, too.
As for the soap, I might add (just a thought...dunno if it'd work) but I'd wash your girth sleeve first with an anti-bacterial soap...not bleach, because that can be caustic, but just like a regular hand or dish soap...I buy the gigantic bottles of antibacterial Ajax dish soap at Walmart for like $3 and use it on everything from dishes to wound cleansing to cleaning brushes. After you give it a wash in an antibacterial soap to kill any microbes that could be causing sores, then you might give it one in your hypo-allergenic...or you might even find one of those antibacterial soaps that advertize they're gentle on hands.