If you did not clean your paint brush properly the last time you used latex paint it may still have paint in it and it will be stiff. To fix a stiff paint brush pour vinegar into a glass container and put it into the microwave until it is hot but not boiling. DO NOT PUT THE BRUSH IN THE MICROWAVE!
Pour the vinegar into your cleaning bucket and thoroughly soak just the bristles for about an hour. Wash the paint brush thoroughly with soap and water and your brush should be in much better shape.
Another option, if you don’t mind the off-gassing, is to simply use brush cleaner and let your brush soak overnight.