history: My dog decided to pee on my bed when I wasn't home. I just cleaned it too! My dog decided to pee on my bed when I wasn't home. Little jerk. Oh no!  And that is hard to get out!  Do you have a waterproof cover on the bed by any chance?
current: Nope. I only discovered it when I tried to lay down on it too. So gross.
next: oh YUCK!  Well, I'd suggest a waterproof mattress cover just in case it happens again.  At least if it does happen again, it won't get into the mattress and you can wash the rest of the bed clothes.  I'm so sorry you discovered it that way!