I think you are right there Mr Bandman. I think that Petunia thought that she had another year of protection or something and was distressed to learn that it would end when Harry was 17 not 18. The reason will be learned when we establish why Dumbledore and Petunia were in contact prior to the death of Lily. I have to say that I do not think that it is something to do with Dudley. I do think that she has had a relationship of some kind with someone in the wizard world other than Lily and James and this has left her embittered. My guess is still Snape who JKR says has been loved by someone. He, I think, is incapable of loving himself he is too much in the mould of his master.