I'd probably have heartburn for a week after downing that monster

[Logatharshini]

hi
There is a conversation between the customer and server in a humburger resturant, what is the meaning of the sentence...
I'd probably have heartburn for a week after downing that monster.
The server : Would you like to try the one-pound super hot and spicy cheeseburger?

Customer:Uh. I don't think so. I'd probably have heartburn for a week after downing that monster.

Please any body help me.
Thank you.

[Mister Micawber]

I would probably have indigestion for a week after eating that very large hamburger.
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Native English teacher at Mister Micawber's