Q: Is it Allowed to Sleep on Stomach?  Why is it forbidden to sleep on ones Belly? Is it forbidden for both men and women?

A:  The reason for this is that it was forbidden by the Prophet (peace and blessings of Allaah be upon him.
Yaeesh ibn Tihfah al-Ghifaari reported that his father said:
 “I stayed as a guest with the Messenger of Allah (pbuh) with those of the poor whom he hosted.
The Messenger of Allah (pbuh) came out in the night to check up on his guests, and saw me lying on my stomach. He prodded me with his foot and said, "Do not lie in this manner, for it is a way of lying that Allah hates.’”
According to another report, the Prophet (pbuh) prodded him with his foot and woke him up, and said,
 “Do not lie like this, for this is how the people of Hell lie.”
 (Reported by Ahmad, al-Fath al-Rabbaani, 14/244-245; by al-Tirmidhi, no. 2798, Shaakir edn.; and by Abu Dawood, al-Sunan, Kitaab al-Adab, no. 5040, al-Da’aas edn. Also reported in Saheeh al-Jaami’, 2270-2271)
This is a general prohibition that applies to both males and females, because the basic principle is that rulings apply to both sexes, except in cases where it is indicated that there is a differentiation between the two.
Side Effecting of Sleeping on your Stomach from Medical Point. 
Sleeping on your stomach forces your lower back into extension, and this jams your facets and shortens your muscles.  It also puts unnecssary strain on your neck as pillows tend to be quite a bit higher than the mattress. The neck gets forced into extension even more so than the lower back, which could leave you with a crimp in your neck when you wake up in the morning.  To avoid these problems, we recommend sleeping on your side with appropriate support, or sleeping on your back with minimal pillow height.