If she's gotten mad at you before for texting her, then probably you're nervous because you've done something that is similar to something that has previously had a negative result, and you're worried about having another negative result. It's an instinctual response - negative reinforcement makes the mind worried next time the behavior is embarked on. It's the way that you train on a more subconscious level (whether training a dog or a child or an adult). So, I'm not saying she's trying to train you, just that you can do the same thing intentionally to train, cause negative reinforcement subconsciously makes the brain hesitant to engage in the same behavior again.
Hopefully, though, a single short occasional text won't make her mad.
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Success is getting what you want; happiness is wanting what you get.