Keep macaroni cheese creamy after refrigerating

I occasionally make Macaroni and Cheese with a béchamel/roux (and tuna, but I still call Macaroni and Cheese). I make the sauce with butter, corn flour and milk and add the cheese later. I use a lot of milk and after I've just cooked and served the dish the sauce has a beautiful creamy/liquid texture.

I cook the sauce in one pot and the pasta in another. When both are cooked I quickly drain the pasta and add both it and the drained canned tuna to the sauce. I then heat it just long enough to raise the temperature of the tuna without changing the flavour by recooking it.

However, when I refrigerate the leftovers and reheat them I just end up with clumps of pasta (and tuna) stuck together with an almost solid sauce. The flavour is still good, it's just the texture that's changed. Is the pasta absorbing extra moisture before it cools? How can I prevent this so I have creamy reheated Macaroni and Cheese?