back pain 4 months after left mastectomy

am a little worried about back pain, had breast cancer ten years ago then again in june this year grade 2 stage 2 2cm tumour hormone rec not aggressive.I think the pain maybe tension it runs down the back of my neck and shoulders and the more i worry the worse it gets.I also have 3 kids and one is only 5 months so i spend a lot of time sitting and bottle feeding and so my posture isnt grt then ,so this must not help.Im worried it maybe cancer in my back,what would the symptoms be??? Ive been on ibrupofen since yesterday ,it helps a little, feel like a hypocondriac at the mo, is this norm??? Am just nearing the end of my fec chemo and will see my consultant nxt wk so will mention it to him.Im hoping the pain is to do with the masectomy.I havent had recon so maybe my backs compensating for the lack of muscle at the front,is this poss??? need reasurance x