I am calm. I am very calm :)

YOU MIGHT BE VERY PANICKED RIGHT NOW BUT IT WILL BE OK it’s better to just tell her now, be very sincere in your apology, explain why you were nervous about telling her, and prepare to explain a plan to get better grades in the future. YOULL BE OK! ITS NOT AS BIG OF A MISTAKE AS U THINK IT IS and in any case it’s not like you can keep those tests from your mom so it’s really better to fess up!! Calm yourself vent acc! It will all be fine. You can probably even be like “oh word nevermind I think they came today instead I must’ve been confused” to get out of the lying part (but it might sound like bs, so being truthful is best and she’ll probably understand!)
good luck vent acc!! your grades can be fixed, and your mother loves you. This will turn out OK!