It does sound like MG. It can clear up on its own, but severe cases can kill a chicken. It such a tragedy that governments have so severely regulated antibiotics that it's often a death sentence for flocks that need an antibiotic but can't get one. There may be ways around this, but it takes...
MG is a chronic disease aggravated by stress. That includes environmental stressors such as weather. Treat the flock with Tylosin powder. You should be able to find it online or at feed stores.