I have flat foot I use customize orthotics made by a podiatrist, why when I inline skate my lower back kill me?

Not the orthotics. It is probably not caused by the orthotics. Since most inline skates come above the ankle and lock the foot & ankle in position the orthotic does not serve much function. It is probably the position you put your back in when you skate that is causing the stress & pain.
? wrong for skate. Skates can be difficult to fit with standard orthotics. Furthermore, the mechanics affected by an orthotic for standing/walking can be different than those used during a skating stride. Skates often require "shimming" between the boot and blade holder to resolve foot mechanic issues rather than orthotics.
Orthotics. Both answers are correct - orthotics won't work well in skates . It is likely a positional problem . Try doing some reverse crunches to strengthen your lower back .