Guaifenesin for allergy/asthma related chest congestion?

I've had asthma and allergies since I was about 14. Every year I get the same thing: Stuffy nose, drainage, chest congestion, coughing, wheezing, difficulty breathing. The works. No real headaches, unless its really bad. I've been taking my allergy medication, but the chest congestion is persistent. However, last night I took some Guaifenesin and it seemed to help. I got to sleep at least. I've heard that people with Asthma shouldn't take Guaifenesin! Is this true? If so, is there an alternative? I don't have a job anymore, meaning I don't have money to go to a doctor!