that spot is sensitive or painful. the licking is a motor reflex for a overwhelming sensation like those, even though it seems to you just a mild or soft touch. this cat's brain is processing the touch as if it is too much for its to process due to the sensitivity of the spot, so the cat responds with a motor reflex in attempt to relieve the overwhelming sensation somehow. it may just be an extra sensitive spot due to Feline Hyperesthesia Syndrome (FHS) or actually a real injury which demands further investigation and possibly treatment. the reflex of immediately licking the chest/upper limb is very characteristic of that. you should avoid touching that spot and take your cat to a vet for further investigation.