There are too many causes to tell you what is causing your problem. Since you are young it is a concern; age drooping is the most common cause in 50 years old and older.
This is a disturbing symptom and a disease called myansthenia gravis needs to be excluded.
See an Eye MD ophthalmologist. Have your family MD refer you or go to www.geteyesmart.org and find an Eye MD there. When you go in take 8 to 10 good facial photos taken of you over the last 20 years so that the Eye MD can see what they looked like earlier in your life. Do not let the technicians put any drops in until the Eye MD looks at the lids.
JCHMD