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your radiation therapy to your right breast should not have been over 3 months. it should have been 5-6 weeks of daily radiation therapy.
tingling sensations like ants crawling is a sign of parasthesias (abnormal nerve sensations) from nerve damage
you should see a neurologist and get a shoulder and neck spine MRI scan with contrast to look at the nerves and look for pinched nerves in your neck or pinched nerves in your shoulder which could be causing these parasthesias.
there are nerve pain medications like Neurontin (gabapentin) or Lyrica (pregabalin) which could help as well.
it doesn't sound like your doctor has ordered an MRI scan for you yet, correct?