The rash you're getting is likely chaffing from the suit no longer fitting you well and not likely an allergy. I base this on the description of where the rash is. I get these all the time whe a suit doesn't fit me properly or wears out and it's just a matter of getting a better fit.

As for sizing issues. Every once in a while I've come across a suit that is supposed to be the right size and is way too long. I had this happen with a TYR suit, I bought it without trying it on because I've bought the same model many times and I know my size. The suit was too long and when I held it up to another suit, the body was a good 3 cm longer than my other worn out suit!! So I would encourage you to go someplace where you can try them on or make sure you find out if you can return it in a sizing issue situation. That's maybe a long shot.