While cleaning,I feel a lot of blisters/bumps on the bottom of tub,going around ,where the bottom meets the sides of the tub.this bathtub/shower is only 8 years old.What could thid be?

What material is the bathtub? Fiberglass, steel, or cast-iron? Back to you, Tom

I'm guessing this is a "bath-fitter" or similar type tub/shower system, maybe?

Mark thinks you have a bathtub liner. Is that the case? Back to you, Tom