Also, consider that there are times when we do forget things, that maybe she had other things on her mind. It happens.
In parts of China, when it's a child's birthday, the child goes to make her/his mother a cup of tea. This is to thank the mother for enduring the pain of child birth, which brought forth their life. That's a twist for you to consider.
(Hmmm, or did my mother make that up LOL!)
I remember, on my 16th birthday, I woke up late and found a note saying "We've all gone ice skating for the day" .. and when they got back at night, I said "Hey, It's my birthday" and my mother went and baked me a cake straight away. It was cool.
So, like Zoebess said, just let your mother know how you feel and I'm sure you can straighten things out quickly. Don't stew on it any longer, just pick up the phone.