At worst its very light flash rust probably. No way it could rust bad in that amount of time. Follow what was said above, and I'll bet if you use a Jag to clean...you'll be good to go.
And, fwiw...the sidelocks I've had over the years I just used HOT soapy water to clean. Stick the breech end down in the sink, and with a jag/patch on your ramrod and suck in the soapy water and pump it back out. If the water is hot enough, it will dry quickly on the inside. Then a light oiling and you are set. Works like a champ, for me anyhow.