Regarding back ache and vanishing symptoms - I slipped a disc in my back the day after ov, so my first week post ov was spent in bed and having physio. About a week after my last slipped disc physio appointments (disc was back in and back much better), I was getting burning shooting pains in my right bum cheek, like someone stabbing me with a hot poker. Went back to physio assuming it was still linked to slipped disc. Told her I was now 4.5 weeks pregnant. She felt it and said disc is fine now, that bum cheek stabbing is an extremely common pg thing! It's the body already starting to adjust and she gets lots of pregnant patients coming with the same complaint!
As for symptoms vanishing - it's happened to me twice now - the day I went for my first HCG bloods (4+4) I had zero symptoms, after having multiple bouts of nausea the previous day. Yesterday (5+1) I again suddenly had zero symptoms after having a very full, pregnant feeling lower abdomen the day before (which was also when I got my second HCG results). It is very disconcerting suddenly being totally symptom free but we have to just believe all is well. Today I've woken up with boobs that feel somewhat bigger. Also, I think my dreaded pregnancy constipation is setting in.
As unpleasant as symptoms are, they really are reassuring.