My mom went to a Christmas party tonight and so I was encharge. So I gave the dogs their mess, my mastiff gets 2 Benadryl and that's what I did, when my mom got home I guess she wasn't thinking and she gave my mastiff 2 more Benadryl, my mastiff weighs about a 115 pounds but I'm scared, what should I do :(

Tell her

Yes, just tell her, it was as much her mistake as yours.

The benedryl overdose will just make your dog ultra sleepy.

You and your mom need to be in better communication with each other when it comes to meds. If this were anything else, it could be fatal.

If your pet is on meds, specifically, you need to keep a log that everyone can access and see what has been given, date and time.,

I agree with Lucky and Tickle. Your dog will likely be okay, this time. But for future you should keep a log. Easy to do. If you have computer skills, make a meds schedule, print it, and then whoever gives a dose signs in on the sheet, date, and time, so that there's a record. That way there's no double doses, and no worry.

If your dog is having a reaction to this overdose it's time to see your vet. Sadly there's very little we would be able to do online.