Hi Jonny,
Your girlfriend is one lucky girl and she knows it. Why? Because she is old enough to know a good guy when she has one and she holds on to you. Not too many women out there are lucky enough to have someone who gives them freedom, is considerate , understanding and loving, all at the same time!
I do not think you have a cause to worry. If she is a little different during the weekends ,it is nothing to worry about. We all act differently when we are away from our daily routine.
You are perhaps feeling a little unsure of yourself lately and insecure but do not frett unnecesarily.Try to relax a little and do things you enjoy so that you focus on subjects other than your relationship.
If a woman tells you she loves you at 39, she must mean it. She is mature enough to know her mind and has the freedom and ability to move on if she doesn't like what she has. But yours hasn't. So, I wouldn't worry about it.