I have been getting a skin bump on my left arm every few months starting this year?

my skin would start to itch and then right after a bump would form where it was just itching... the bump always form in the exact same spot... this is the very first year this has started happening and it happened about 3 or 4 times already... I'm not allergic to anything and I havent had any skin coniditon that I'm aware of..the bump comes in an instant but within 30 mins the bump would just go away all of a sudden ... do any one know what this is