Getting a yeast infection doesn’t mean you were doing something bad, so you shouldn’t be ashamed to tell your mom. It could be caused by a lot of different things. I’m big on open communication so I think you should just tell her. While treating yeast infections can be pretty simple, it’s always best to see your healthcare provider to get the correct diagnosis. I’d tell her and then work on getting it looked at. Always remember, there is no shame when it comes to your body. Don’t be afraid to speak up!

I’ve been trying to figure out why you would have a hard time telling your mom so I finally pulled my 15-year old daughter into my room and asked her if she would tell me if she thought she had a yeast infection. 
“It would be awkward,” she said. 
“But why?” I asked. 
“We’re done,” she said.

Before she left she did admit that she would tell me, but she wouldn’t feel comfortable “talking about it.”

I guess I get it.  It can be embarrassing.  However, if you are in pain or discomfort you need some medical attention.  You shouldn’t suffer or let a possible infection linger.  And I’m sure your mother wouldn’t want you to suffer.  Try this: say, “Mom, I think I have some kind of infection and I’d like you to make me an appointment as quickly as possible.  But I’m uncomfortable talking about it with you, so please just make the appointment.”  If your mom is like me she will probably try to make you talk about it.  At this point you can just say, “I promise to bring up all my symptoms and not be shy while talking to the doctor, but I don’t want to talk to anyone else.”  That’s what would work with me.

I think instead of telling her you think you have a yeast infection, just tell her that you're really itchy down there and have some goopy stuff in your panties. Yeast infections happen to most women at one time or another and since they have nothing to do with sex you don't have to worry she's going to think you have an STD. If your mom is really modest or embarrassed when you talk about anything like this, why don't you try writing her a note explaining what you think is going on, asking her to make you an appointment with your health care provider. Good luck and I hope this helps!