Let it go and do nothing more. What you are experiencing is what is called "unearned guilt.

You were honest - you were busy so work. You texted to thank her for the gift. Nothing more is required. It may not feel right that she did not reply but she is allowed to behave in any manner she chooses. Now you can elect to do the 'work' of letting go of the issue and not make more of it than it is. You did nothing wrong.