A red welt the size of a silver dollar on my lower back it's got a white center & itches & a hard knot underneath when i rub it. possible causes?

Welt: This may be an insect bite in view of the firm swelling in the middle along with redness and itching. A giant hive would be another possibility. A hive usually resolves in a day or so and others may appear as part of an allergic reaction to something. If it's a bite it takes longer to resolve, but if redness extends have it examined for possible complicating infection.