Frozen BM smells bad

GEM529

DD is 5 months old and I have been super lucky that I have not had to use my frozen stash that much.  The few that I have defrosted that are 3-4 months old smell bad.  Not soapy or metallic, but like milk that has spoiled.  This is milk that was frozen right after pumping it and it has been frozen the whole time.  Do you think 3-4 months is too long?  Should I be using it sooner?  I know that Medela's chart says it is safe at 3-4 months.

 I don't remember having this problem with DS.