Retrograde Ejaculation: This condition occurs when semen enters the bladder instead of exiting through the urethra during ejaculation, often due to issues like diabetes, surgery, or certain medications. The sudden decrease in semen volume and its watery consistency could be indicative of this condition, as the semen is being redirected into the bladder.

Hypogonadism: Low testosterone levels can affect semen production, leading to decreased volume. This could be due to various factors, including hormonal imbalances, testicular issues, or pituitary gland problems.
Ejaculatory Duct Obstruction: Blockage of the ejaculatory ducts can prevent semen from being ejaculated normally, leading to a decrease in semen volume. This could be caused by infection, injury, or congenital conditions.
Prostate Issues: Conditions like prostatitis (inflammation of the prostate) or benign prostatic hyperplasia (enlarged prostate) can affect semen quality and volume.

Testicular Cancer: Although less common, testicular cancer can lead to changes in semen characteristics, including volume. Early detection is crucial for effective treatment.
Neurological Disorders: Certain neurological conditions, such as multiple sclerosis or spinal cord injuries, can affect the nerves involved in ejaculation, leading to changes in semen volume and consistency.

Young's Syndrome: A rare condition characterized by obstructive azoospermia (blockage preventing sperm from mixing with semen), chronic sinopulmonary infections, and bronchiectasis. It could potentially lead to changes in semen volume and quality.
Cystic Fibrosis: This genetic disorder can cause congenital absence of the vas deferens, leading to ejaculatory duct obstruction and subsequent decrease in semen volume.
Medication Side Effects: Certain medications, such as those used for depression, high blood pressure, or cancer treatment, can have side effects that include changes in semen volume and quality.