Hi both of you,
I have had similar problems which although no infection showed in sample turned to kidney infections on two occasions! Really really nasty, was ill for a few weeks, high temps etc etc. I have also had numerous cystitis attacks some of which showed infection, showed didn't. All this started the week of my 60th birthday.
Had numerous tests, kidney scans, seen urologist etc etc.
Cutting a long story (3.5 yrs of problems) somehwat shorter, it has all turned out to be vaginal atrophy and has been much improved (I don't like to say cured!) since I went on topical oestrogen therapy. As you dry out 'down below' you lose the natural bacteria that protect you against infection. Nobody, gp or urologist explained this to me
Only found it out by reading Menopause Matters and I now use Vagifem pessaries.
Do hope this is of some help to you, I really sympathise with what a horrible problem it is, made me very very low.