The next time, put a little handful of instant potato flakes into the sauce. Wait a few minutes, and if it's still too thin, add a bit more. You won't taste them, but they are a wonderful thickener.
Clare is spot on with the instant potato flakes. You can't really use pasta to thicken up a sauce as it may end up breaking down the pasta and you end up with a gummy yucky concoction, especially with slow cookers.