Thanks for all the replies, it gives me a lot to think about.
I did not discuss this with my mother, this is something we both thought of independant of each other, so she isn't technically "stealing" my idea. I however, wanted to get it for her as my mom is already buying her other things from the list I did give her, as well as additional items. I wouldn't discuss with my mother what I was getting her, as she does tend to be this way.
I do think I'll try and let this go for now, but I'm definitely going to talk with her after Christmas. I've been stressed and upset by quite a few issues involving my mother, and never like to talk to her because she always tells me I'm being ridiculous, or pulls a "well if you're going to be upset with me, I'm going to be upset with you back" thing.
This one thing probably isn't such a big deal on it's own, but it's small things like this that build up.