I had the same problem with Old Spice. What are you using now?
I'm using Arm & Hammer Ultra Max antiperspirant/deodorant.
They also have the same exact name, "arm & hammer ultra max" that is deodorant only, but it doesn't work nearly as well as the antiperspirant/deodorant.
I used to think I couldn't use antiperspirants anymore because of the rash I was getting, but found out it was from something that some companies use in their scent. I've never had an problem with Arm & Hammer since.
I started wearing "under armour" type shirts to stop hyperhydrosis (excessive sweating). The wicking effect of the under armour helps prevent pit stains.
I never had this problem and it is crazy hot where I live but I will tell you that you will sweat far less wearing cotton than any type of blend.
Also, make sure that you are totally dry before you put the antiperspirant on.
I still think it's body oil since it's on the back waist line too. I sweat a ton...salt stains and all, and I always use deodorant with antiperspirant and get the pit stains, but not the stains on the back waist line. I do have an oily face and I do get discoloration of my neckline if I'm not religious about washing my face frequently.
To get out oil, it really helps to wash in warm to hot water, with detergent of course. If you use bleach, give the detergent some time to work before dumping the bleach in.
Holy crap. You shower too much.