My lining is thin again.

I don't understand it and neither did the tech because I haven't had any spotting so where did it go? The past two monitoring appts. it was around 7mm. Today it was 4.38. The tech said she didn't want me to worry about it because at cd10 it isn't a concern yet. I like this tech a lot and she said she would tell me straight out if she was concerned at all, but I am still worried about it.