Shadowing has suddenly appeared

I have been editing flowers for over a month with absolutely no problem and all of a sudden I am getting shadowing appearing when painting back in on a mask layer. The only way to avoid is to leave a large gap around the edges which is totally useless and before I could paint right up to the edges of leaves.

my process is

1 open photo

2 overlay a texture background (using pace embedded as Jo other way currently works with my version

3 Add a mask layer

4 set opacity of overlay to around 40%
select mask layer and paint over parts of flower I want to see - before I could go right up to edge of leaves with no problem

5 select texture layer and adjust opacity until desired effect

worked perfect until today and now if I go anywhere near the edges of the flower, a shadow appears outside the edge and on the background texture :(

any tips on what I might have accidentally switched on would be gratefully received

thanks in advance