The environment hasn't changed, but it's not in an ideal environment (if that makes sense - it's been this way for years). So that may be a factor.
Also what might be a factor is my inability to actually look closely - I can "remember" restringing and it being OK, but when I looked at the G string last night (no sniggers at the back, please), it's truly awfully wound. So that's probably my problem. Don't I feel stupid?
Seriously, I appreciate the replies here and I've learned a lot (including the luthier's knot, which I'd actually been doing by chance, just not very well!).