I tried the dillution method waterbear suggested, by taking 1 part pool water and 3 parts distilled water. The CYA then came to 35, then multiplied by 4, so according to this reading my CYA is 140. This is way more in line of what I would've expected after the water change. I'm still puzzled by the pool store's reading, but it makes sense if high CYA becomes inaccurate to read.

Thanks everyone - I'm taking my first "test-swim" this weekend, in 10ppm of FC, so I'll let you know if the initial problem, the rash, is solved....if I don't return though...I hope it's not from Chlorine Intoxication