Hello, I have been working with couples (many with sexual issues) in therapy for over 20 years. I am happy to answer your question. There is no medication a woman can safely take to increase her sex drive. The male hormone, Testosterone, does increase female libido, but it has serious side effects and is not FDA approved. Some medications, like anti-depressants decrease sex drive. If this is the case with your gf, she should see her doctor and ask to switch to an anti-depressant that does not have sexual side effects.
I am pleased that you are considerate of her feelings and do not push her for sex when she is tired from working. There are some things you can do to increase her sexual desire. There are creams and gels available over the counter that will make sex more pleasurable for her. Also most women need attention and affection and romance to become sexually aroused. So take her out for a romantic dinner, buy her some flowers, leave little love notes around for her. And since she is tired from work a lot, give her a lovely massage without expecting sex. She may surprise you!
I hope this helps. Chat back if you have any questions. Take care, Eleanor