It sounds to me like the pool was "let go" (i.e. not chlorinated) over the winter and that bacteria have converted some of the CYA into ammonia. That would lead to a huge chlorine demand upon opening and initially any chlorine added to the pool would form monochloramine which registers as Combined Chlorine (CC). I describe this technically in this post and describe my own experience with this in this thread.

The bad news is that if the bacteria truly converted around 60 ppm CYA into ammonia, it would take at least 150 ppm FC cumulatively added to get rid of. Hopefully, some of the ammonia outgassed so that the actual amount needed won't be that high, but it will take a lot regardless. So keep adding chlorine with the pump running adding it over a return flow in the deep end and measure the chlorine level after 30 minutes. If the FC is still low, add more chlorine. The CC may continue to climb, but keep adding chlorine until the FC starts to rise. At some point you'll reach breakpoint and the CC will start to drop.

Do not add any stabilized chlorine at this point. A higher CYA level will only make the process of getting rid of CC take longer. Do not worry about the pH and TA at this point either. Having the lower pH to start with is fine since shock levels will tend to raise the pH anyway (at least temporarily when the FC gets high)

To prevent this problem in the future, maintain chlorine in the pool over the winter. If you live in an area where the pool freezes over, then shock the pool with chlorine when it gets below 50ºF and then open the pool up again before the temps get above 50ºF. Chlorine usage is very low at colder water temps.

Richard