The expiration date is only a guide. Sometimes (not very often I might add) the milk will go bad before the expiration date. Other times it could be a day or two after.
The rule is simple: If it smells like milk, it's fine. If it smells nasty, cheesy, or not quite like milk should, it isn't. Trust your nose.
Never leave milk out. Always keep it in the fridge. Also, never risk two days of straight vomiting just to save a few cents on a bit of milk.
Lastly, you posted this in mental health which is not an appropriate category for this question. Please review your choice of category next time.