Milk came out of my belly button what does thid mean?

Fistula: If fluid is continually leaking from your belly button you could have a fistula (tunnel) between it and your bladder or abdominal cavity. You should consult your physician. If it happen one time and you happened to be drinking milk you may have just spilled some into your belly button.

Urachal cyst: If u have or had a piercing that is likely cause, but fungal infections or congenital pockets called urachal cysts are very common also. These often present with varied severity and recurrence; pus (milky even stinky smelling discharge) may be present at times +/- pain. Often topical care is effective but if u r very bothered, surgery is often effective. An exam +/- ulratrasound can confirm.