A photo would be useful, but you obviously have a small leak, it doesn't have to be much, even an egg cup full of water could spread to half of your mattress - assuming you've had rain like we have, there's a lot of chance for it to get through.
I suggest you get some dry cardboard and put this on top of the mattress, you'll soon be able to see the discolouration and be able to see where the water is coming in from. If the rain stops, use a hosepipe (assuming you haven't got a ban) and have a good look.