She's probably not only upset that you guys were gone, but she's reaching the age where she's going to purposely urinate outside of the litterbox. It happens when they reach sexual maturity. The only thing that will change that is getting her fixed. I hope you were planning on doing that..
I mean, you can try essentially re-litter box training if you think it might do some good, reward her with treats and use positive reinforcement when she pees in the litter box, otherwise, she really does need to be spayed.