Em.
If you go for a BT... try do it closeish to CD20.. i was lucky to be on CD20 when i had mine as my GP said its done on CD21... so i was able to get it done. She was able to tell me at the results that i was approx 10DPO.. which was close to spot on.. that was due to the hormonal values in my blood.

I also had a relitively pain free O this month too... only a small amount of rounded dicomfort in my pelvis... but i had pos OPKs... otherwise i wouldnt have thought i O'ed either.

I hope you work it all out Em!!!

keep us posted.