This morning at 11am i had a 710ml monster energy drink and at 9:30pm, i took two melatonin, will i be okay?

Yes: Yes you will be okay. If you are generally in good health. 18 years old and with no history of heart disease, you will be alright. However this is a good opportunity to point out the dangers of high energy drinks used in excess. They can cause hugh blood pressure, heart palpitations, chest pain and anxiety symptoms. Also trying to counteract the effects with melatonin is not a solution.