She may have gotten mad that you didn’t wish her a happy birthday because she has romantic feelings for you and was hurt that you didn’t remember her birthday.
You may believe that this shouldn’t be a big deal. After all, you are not in a relationship with her.
Girls often use a period of casual dating as a testing ground for how the guy could be as a boyfriend.
She isn’t just casually dating you without any thought. She is doing it for a reason.
Often, the reason is that she is testing you out.
She is observing what you say and do and trying to interpret what that says about you.
How would you act as a boyfriend? How would you treat her as a girlfriend?
These are questions she has in her head the longer your casual relationship goes on.
In your mind, you may believe that what you have with her is light and commitment-free.
However, the longer you are in this kind of relationship with her, the more likely she will begin to judge your actions based on what she would want you to do as a boyfriend.
She would become more attached and expectant.
This is what typically happens when casual relationships go beyond the one night stand and enter into weeks or months of hookups.
Feelings will inevitably begin to develop.
This is partly what makes human beings affectionate creatures.
Human beings are not robots.
A person can only go so far with someone else before they begin to develop some kind of feelings.
This may be what this girl is going through.
She may be hurt that you didn’t wish her a happy birthday because she is already looking at you as a possible future boyfriend.
She may have already fantasized about how it would be. She may already feel like she knows you.
Now that you haven’t wished her a happy birthday she is upset that you have ruined that dream.
You may have ruined what she thought she had with you as far as feelings go.
Girls want to feel wanted and desired.
They want to know that they are special and be reminded of this on a regular basis.
This is often shown through affection, compliments and attention.
She likes to feel confident in the fact that you care.
This makes her feel special and wanted.
When you don’t wish her a happy birthday, she doesn’t feel special.
She doesn’t feel like you care.
She doesn’t feel like she is being paid the right kind of attention that she deserves.
You may not be in a relationship with her, but she doesn’t see it that way.
She sees you as a very likely partner in the near future and you have just disappointed her.
This is most likely what she is thinking and how she is reacting to what you have just done.
She may have gotten a happy birthday wish from everyone else in her life who cares about her except you.
This is what could disappoint her greatly and make her feel hurt. She wants the attention and you didn’t give it.