I would wait for Ben's test kit for your CYA level. Unless you are using a large amount of stabilized chlorine I can’t see 5lbs of CYA getting you to a level of 90 that fast. CYA is slow dissolving and can take a week to reach its full level. Since you poured your CYA in the skimmer I would also recommend you not back flushing your filter for a few days. The CYA will stick around in the filter while it dissolves.
The role CYA plays is kind of like sun screen for your chlorine. You go out in the sun you want to be protected but you still want a tan. Think of chlorine as your skin you want your skin exposed to the sun to get a tan but you don’t want it burnt. The CYA protects chlorine by bonding with it to protect if from the sun but we still need it to interact with the water. When chlorine hits water it forms other compounds that do the work of cleaning organics and killing the bad bugs. Like too much sunscreen too much CYA will not allow interaction of chlorine to the water in the pool.
I would be cautious of pool store results and wait until you get Bens kit to really dose your pool. You might go to Walmart and grab one of the 5 way test kits with liquid reagents while waiting. The CYA test seems to be a good one but be cautious about the TH. If you do get one of these kits remember some of the test are not accurate with a chlorine level above 5.

May I ask where at the pool are you getting your water for the test?
If you poured your CYA in the skimmer and getting the water at the return it may account for the incresed CYA level.
Steve