Chihuahua Is Coughing Up Nastly Stuff

I am new to this dog thing, I dont know why, even though I take really good care of her, she still seems to be acting weird. She is 2 and a half years old. She woke up this morning and was fine and throughout the day she did not do anything until tonight. At night she started coughing and after coughing she threw up some white precipitate stuff. It looked like human spit.  Can someone please help me with this problem????