I Have Severe Dizziness Comes And Goes Some Really Bad

i have severe dizziness comes and goes some really bad some mild,sometimes items in room spin around other times i feel i am spinning.have been to doctors and they say allegies...i had breast cancer in 2001 and recurrence last year with tumor on my sternum.had 17 rounds radiation and am on exemestane to block hormones from causing more tumors.doing good with this but ihave suffered with this dizziness and vertigo prior to this...can you offer any suggestions to rid this spinning..miserable