Often occurs after a big night on Methamphetamines or MDMA (ecstasy). The size of your penis will shrink to a childlike state (<5yrs) after excessive consumption of either of the stated stimulants/substances.
Don't worry, it will return to normal size within the next few days; the length of time probably depending on how much of the drug you have taken.
"Fuck, I can't show her this--I feel like a toddler when I look in the mirror. I got meth dick bad."