A mole on my arm got bigger but then smaller again. It's slighty red around the edge. Should I be worried? (Photo)

You should definitely see a dermatologist to have your mole examined.  Not all changes are necessarily bad, but it should be checked. 

To be safe, you should be carefully evaluated by a
board certified dermatologist. Please use the ABCDE's as your guide for when to be concerned about moles: A for Asymmetry; B for irregular Borders; C for black or uneven Colors; D for Diameter greater than a pencil eraser; and E for a growth that is Evolving or changing. If necessary, it is an extremely
simple five minute procedure for a dermatologist to remove the mole using
either a "punch" or a "shave" biopsy which typically leaves
a minimal scar. The mole can then be checked microscopically for reassurance.

When moles change size they need to be examined in-person by a trained professional. There does not have to be itching or burning present. It is best to make an appointment and see someone for an in-person exam and proper diagnosis. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."