Most times it's just condensation, happens more often during this time if year since its cooling down. To be safe just check you coolant level, look for oil mix in coolant
Normal, you will have water condensating inside the exhaust pipes until the entire pipe is hot enough that the water stays as vapor and the condensation dries up.
In cars that are used for short trips the pipes never heat up and never really dry up - these then tend to rust from the inside.
As bad as water in your exhaust sounds, it could be a lot worse.
Top fuel drag cars run on 90% nitromethane which results in exhaust containing nitric acid - highly corrosive and breathing it in can cause a reaction leading to suffacation.