Low Sperm Count or Oligospermia: A sudden change in semen consistency and appearance, such as becoming watery, could be due to a low sperm count. This condition can result from various factors, including hormonal imbalances, testicular issues, or blockages in the reproductive tract.

Infection or Inflammation: Infections like epididymitis or prostatitis can cause changes in semen appearance, making it more watery. Similarly, inflammation in the reproductive tract can lead to alterations in semen consistency.
Hormonal Imbalance: Hormonal changes, particularly in testosterone levels, can affect semen quality and consistency, potentially leading to watery semen.
Blockage or Obstruction: A blockage in the epididymis or vas deferens can prevent sperm from mixing with seminal fluids, resulting in a watery appearance.
Frequent Ejaculation: Frequent ejaculation can temporarily reduce sperm count and change semen consistency, making it appear more watery.

Testicular Cancer: Although rare, testicular cancer can cause changes in semen appearance. It's crucial to rule out this possibility due to its serious implications.
Prostate Cancer: Prostate issues, including cancer, can affect semen quality and should be considered in the differential diagnosis to ensure timely intervention if necessary.

Retrograde Ejaculation: A condition where semen enters the bladder instead of exiting through the penis, which can result in a watery appearance due to the lack of sperm.
Semen Allergy: A rare condition where the body reacts to its own semen, potentially causing inflammation and changes in semen consistency.
Medication Side Effects: Certain medications can affect semen quality and consistency as a side effect, though this is less common.