Toddler throwing up after milk?

My son is 18 months old and has never had a problem with whole cows milk before. All of a sudden today he has thrown up right after drinking milk. I give him milk 3 times a day at breakfast, lunch, and dinner. He has thrown it up every time about 10 minutes after drinking it. He has also had terrible gas today. The milk hasn’t gone bad at all, I checked it. Do you think it’s possibly just a stomach bug? Any thoughts or advice would be greatly appreciated.