Answer: Without seeing the leaf, I can only describe the two possible causes - you'll need to further inspect the problem to determine which it is. The bump on the underside of the leaf can be fluids from the tissues which escaped because of an injury (almost like bleeding from a wound). The sugars in the fluids can produce a hard little bubble. Since there are circular spots on the upper surface, it indicates an insect was feeding and what you are seeing is the end result of the damage. The pest may be long gone, so no control is necessary. (It's possible it was treated at the nursery before you purchased the tree.)
Or, the raised areas you see on the undersides of the leaves could be scale insects. Theses, too, feed on fluids within the tissues which would leave a discoloration on the upper side of the leaf. Scales can usually be peeled away.
For a positive diagnosis, why not take a few of the affected leaves to your local Cooperative Extension office, or even your nursery? They can look at the problem under magnification, identify it, and suggest control remedies.
Best wishes with your orange tree!
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