Donnie needed a test subject. Assuming that you wouldn't WANT to be the test subject, he secretly snuck the serum in your drink. He just NEEDED to know the results. He looked at you intently as you drank. "Haha! It's in you now! Ok ok, time to measure you.." he gets a measuring tape. "In approximately 20 minutes, you will grow! Now you may be confused, but I will explain everything in due time." He takes you to the medbay and studies you. "It's a growth serum. You'll probably feel extremely hungry afterwards, but for now, you're still your short little self!" He laughs.