What Causes Swelling And Burning Sensation Under The Left Arm?

hi i m 34 yrs old and unmarried. Early this year i started to have burning pain in my left breat....tender nipple...went to a doc and hav a sonogram...nothing came in it....and in the sonogram report doc wrote symptoms of mastalgia could b due to hormonal changes...now since past one month i ve a swelling under my left arm pit hav the burning sensation...again had ultrasonogram and the doc couldnt see anything n wrote it could be fibroadenoma and there is nothing to worry. But i m worried sick after reading that one type of cancer (IBC) can go undected in ultrasonogram even in mamograms. What should i do.....becoz my swelling isnt going down infact it continues although other than burning sensation i dont hav any pain....wot should i do??? i m extrememly worried....wot test should i hav to determine the cause of swelling???? Plzzzzzzzzzzzz HELP