1999 Cadillac Deville Getting trans fluid in coolant

Got my coolant changed and car was running smooth- I noticed it would spit out coolant through the overflow but the car was not over heating. It did not do it for a week or so then it did it again except this time it had transmission fluid mixed in with it. Service engine light came on with 2 transmission codes - what could cause transmission fluid to get into the coolant. And what type of fix would this be? If it is not transmission fluid I am afraid its the head gasket and its oil, but its a redish color so I think it is transmission fluid. Any ideas would be appriciated