If your CYA level dropped over the winter, then if the FC had gotten to zero then soil bacteria in the pool could have converted some CYA into ammonia. That creates a huge chlorine demand upon opening and initially the added chlorine creates CC which is what you have been seeing. Keep adding chlorine until it starts to hold. It may take a lot. You can do a bucket test if you want to figure out roughly how much it will take where 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC. Technical details of what happens is in this post while my own personal experience with this is in described in this thread with a summary in this post.