Hi Paula,
I guess you should consider yourself lucky that she didnt flip out. It seems as though you dodged a bullet this time. It's obvious that she knows something is going on so why not sit down with her and come clean, unless she is trying to lure you into a trap, there is no sense in trying to be clever, thinking that you are hiding when in fact you are not.
Just be aware that when she knows the real reason her reaction may be different. Not knowing the relationship between you and your wife, I have to recommend that honesty is the best way to go, as relationships built on, lets say, untruths, is not a healthy one.
The premarin that she found is it a prescription from your Dr, as she may have realised what your doctor visits were about?
Just my thoughts.
Steph