This is Dr. David
if you are masturbating more frequently you can have less ejaculate volume.
are you masturbating more frequently now?
you could be having some narrowing or stenosis of your penile urethra as well.
do you have any problems with urination?
you might need to see a urologist and get an examination and blood testing to check your testosterone level
have you had any problems with libido?
you might benefit from seeing your GP and getting your testosterone levels checked.
but most likely they will be normal.
change in semen quality can happen in life
sometimes due to your diet or how much water you drink.
usually it is not a serious problem