Howdy ShadowSteel18,
Yup, used up all your RAM. Or rather, the game used up all the RAM its capable of using 3Gb.
I think you diagnosed the reason it happened however. Too many reloads in a short period; and the Chicago Racetrack route is a graphically intense route.
I'm impressed that is the first time you've seen that message?! I can't go one day without getting that message atleast once!
Anyway, a simple restart of your computer or one of those cool RAM cleaning programs should solve your woes.
Take care!