I just drank a monster & took 4 benadryl (diphenhydramine) tabs. what will happen? is it bad?

Why did you do that?: I can't help wondering why you did that. As an energy drink Monster has caffeine and other stimulants so may keep you up for several hours (though this depends on how sensitive you are to caffeine). Benadryl (diphenhydramine) is an antihistamine that makes people drowsy as a side effect. So this is like driving while pressing the gas pedal and brakes at the same time. You will survive but may feel a bit strange.