Do you normally consume that much milk? Have you added anything to your diet that is novel?
If the answer is no to both, you might want to look at digestive enzymes, probiotics, and betaine HCl as the extra intake may be overwhelming you stomach.
One’s stomach can get out of whack quite easily, especially when under stress and being force-fed. The products I listed above will help normalize digestion.
Garden of Life’s Omegazyme and NOW’s Super Enzymes are enzyme blends. NOW’s Gr8-Dophilus is a probiotic bacteria blend, and NOW’s Betaine HCl is a high dose Betaine HCl product (the super enzymes also has 200mg betaine HCl per capsule).