When will my upper arm feel normal after mastectomy with axillary clearance?

You said that you have had it assessed by a physio - is this someone who specialises in lymphoedema?  If not, find one who does.  The tightness you describe could be an early symptom or it could just be the way your arm feels now.  This is something you will have to get used to.  However, do see the physio about cording, regardless.