Frankly you need to speak up and tell her to stop using you and start treating you like an actual partner in the relationship. You should tell her openly and honestly how you feel about her girls trip, and be prepared for her to break up with you because it isn't really a girls trip, she's actually into someone else and is sneaking off to sleep with them. Were she treating you normally and going off on a "girls trip" I'd probably tell you to believe her. But if she's distant, using you, and going off on a girls' trip it's more than likely she's talking to someone else behind your back and doesn't want to lose your help/financial support because she's selfish.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."