I agree with DeliDog. Call your vet and explain what is happening. He/She will be able to help you.

A few weeks after we adopted Drake, he started coughing and hacking up some nasty stuff. I freaked out. I called the vert and he told me what to look for and check on Drake (Color of gums, what he is coughing up). Turned out Drake had a little cough. He told me to give him 2 drops of regular coughing medicine for 5 days. Within the first day, Drake had already shown improvement!