What does it mean when your Chilean rose hair tarantula gets little bumps and bald spot on the back?

The baldness of the tarantulas abdomen basically means that it will soon molt which is called pre-molt. Another sign that a tarantula is going to molt is an increase of size in the abdomen. If your tarantula hasn't molted the baldness could also come from the urticating hairs that it kicks when it is defending itself. Hope that helped.