Dear Monika Saliva drooling out of mouth during sleep is normal to an extent and it usually had a frothy aappearance. If it exceeds a certain limit , which you certainly feel is abnormal ( with a very large area of your pillow wet), it might be due to any condition causing neuromuscular weakness of your facial muscles specially around your mouth causing increasing salivation.since you mentioned pretty sure that it s not drooling and it s a foamy discharge one can presume presence of an oro nasal communication or an oro-antral communication or certain throat conditions. Also in cases of history of trauma and road traffic accidents one can have csf discharge through nose which leaves tram lines on the pillow .Please get an ent consult to rule out the above . Hopefully this could be if some help to you. take care .