Please I need your help! My Chihuahua is 3,5 months old and one of her previously standing up ears is suddenly folding down. I have no idea what may have caused it and how to fix it. Thanks in advance.

This often happens when they change their teeth. Make sure her diet contains all nutrients necessary for a pup in teething period.

It's because she's teething. Her muscles are using all the energy for the teeth to grow, so she is too weak to control her ears. Don't worry, it's totally normal. In a while her ears will go back to normal once she finishes teething