Believe it. It happens. You need to confirm it is the pump - as has been suggested, there is probably a weep hole on the bottom that drains coolant when the seal fails. It's just a manufacturing thing, some times you get one that last a lifetime and sometimes it lasts a couple of months. I had one fail after a few months before, but it had a lifetime warranty so they replaced it for free.

__________________
'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
______________________________________

"Back off, man. I’m a scientist” ~ Peter Venkman