Had this big bump on my back. Recently this year, I contracted small bumps on my shoulders, arms, and upper back. Now they are on my legs. What is this?

Without an exam... ...There is no way to sort through the many possibilities. Let a dermatologist examine you and give you a precise diagnosis.
Bumps on Body. It is very difficult to advice you what these bumps are, without knowing the size, the colour, the feel, and any other associated features and conditions.These may be benign cysts or sme benign tumours.But i will advice you to see your family doctor and let him/her check you and advice you what exactly, it could be.