Oh lovely, it's going to be ok.
At the very least you really do need to talk to a medical person, they will help you and check if everything is OK. They won't tell your mum if you don't want them to - they aren't allowed to.
If you can tell your mum then that would be for the best, but if you really feel you can't then it's up to you, you know her best. I'm sure she loves you, and if she's cross it will only be because she's scared too.
If it's too hard for you to tell anyone can you ask your friend to tell her mum, while you wait outside? Then you can talk to her but you don't actually have to say it out loud to start the conversation, would that help?
Or do you have an auntie or a granny that you can talk to? Someone that can tell your mum for you if you're too sacred?
It does sound like you must be quite close to the baby coming, I don't want to scare you, but it's important you tell someone medical as soon as possible so that they can help you. They will be kind, it won't be the first time they've had a young patient.
I've known girls as young as 13 have babies, it happens more often than you think.
If you phone 111 I'm sure they will tell you the phone number of the midwives or the hospital that you need to speak to.