For the first couple of months of a relationship, both of you will be trying as hard as possible to present your best side. You use up all your best jokes and wear those shoes your mom forced you to buy, and she puts her best dresses on and spends hours on her makeup. After that, well, it's straight to comfort city.
After about half a year of dating, you don't need to impress anyone anymore. You're already in a relationship, so you can go back to being the flannel-wearing slobs you always were. That's why any new changes in her appearance should set off the alarm bells. Any new clothes, and sudden obsession with doing her make up again, and she's more than likely trying to impress a new man in her life.