With recent negative investigations and known vaginal atrophy, I personally wouldn't rush to the GP with a bit of spotting. 
I would simply increase the vaginal estrogen, using it daily if necessary, and adopt a watch and wait approach. 
If it is persistent, doesn't respond to improved estrogenisation of the local tissues or becomes heavier or associated with symptoms such as pain or offensive discharge, then I would see your GP. 
I am also assuming your cervical screening is up to date. 
Gynaecological cancers that present with bleeding typically develop and progress over years, not days or weeks, and I personally believe the sense of panic and urgency with which every little bit of spotting ends up being referred as "urgent suspected cancer" does a lot of harm.