Can a normal brown mole that turns black with an irregular red border overnight be all of a suddden melanoma? could it be from a rhogam shot the day before in the same arm?

Have it checked: Unless the injection was directly into the mole, an injection in same arm at another site should not have caused the changes you described. Please have it checked asap.

Mole: A cancerous mole does not change its color or becomes irregular just overnight. If a mole all of sudden change its color and becomes more irregular just overnight- an infection or inflammation has to be ruled out- perhaps trauma etc. Or perhaps, it has been there but you just noticed that now. I doubt that Rhogam shot will cause that.You should see a dermatologist and have your mole checked.