When I’m with my girlfriend she will ask me to give her a massage or cuddle with her or something and every single time she tells me to stop touching her and goes to the other side of the bed turned the other way. Every single time I ask what is wrong and every single time she tells me that she is over stimulated. For the rest of the time she just sits there all grumpy and doesn’t talk to me or anything. She has always done this and I’m wondering what this could be.
It sounds like she's either conflicted about her feelings or uncomfortable in some way. Try talking to her about this in another setting away from the bedroom and calmly invite her to elaborate on what she means.
It could be that she wants to feel closer to you but once you start touching her and making her feel good it kinda scares her or makes her feel uncomfortable. Talk to her about it in a comfortable environment, preferably any from the bedroom like Jonathan said and ask her to explain what she feels and offer your support.
To be over stimulated is a term people with high anxiety use to say that they're feeling too charged in a situation. Generally it means the touching is sending too many signals to their brain and they need some time to relax. You may want to address with her if she's really ready to be as intimate as you are.