Bob says - strangles, especially if she has difficulty breathing. She most probably never completely licked it last spring.
Now to check the vet manual.
She's breathing ok. She only had to stop that once in the pasture to cough that stuff out. Mine didn't have that strangling sound and problems breathing when they had stangles because they ruptured but they would have to stop and cough junk out with any exerction including walking. If it's a relapse it's very mild or she's in the early stages.
Her throat was very tight yesterday. If it's illness she may have fixed the problem herself.