advice on what this means?

Sit down and talk to her. Be clear with her that if this was a joke it isnt funny. You take your relationship very serious and if she is feeling differently then its only respectful enough she tell you rather than leaving hints. Sometimes you have to be bold and have these conversations. I've never played and told my husband i dont love him anymore. It's been said in joking there is always some truth. good luck

No one says that without meaning it. She was probably smiling because she herself didn't understand how she could marry someone, be very much in love with them, and then it dissipates.

Get divorced sweetie; she's decided to move on.

Who knows? This is very hurtful but I don't think she is challenging you.