What Causes Rubbery Mucus While Blowing Nose?

hi! i went to the doctor on the 31st for an upper respiratory infection and got a z-pack of antibiotics. I'm still somewhat coughing up stuff and when I blow my nose I occasionally get these solid strings of green mucus, they're almost rubbery.. I'm currently taking Allegra-D since I have seasonal allergies. It's helping to get the stuff out but I was curious as to why the mucus seems to be so rubbery! this has never happened before