First of all, at 25 you are starting to mature and your body's hormone levels are falling off. This is a natural progression of events and will naturally lead to loss in ability and desire, slightly. I used to masturbate or have sex 3-4 times a day. Now I do it once, and I'm 28.
Secondly, your problem may be stress related. Depending on your occupation, see your boss or manager, tell him that you think you have too much stress. If you are in a position where that is not possible, seriously consider a change as other health conditions that exist because of too much stress - higher blood pressure, hypertension, greater risk of heart disease later on, weight gain, premature aging, ... the list is endless.
A short term solution would be to take a month vacation. Don't worry about sex, girls, etc. Do something else that is gratifying to you. Go outside and play. Go for a cruise in your car to someplace you've never been. Meet up with the buds and shoot some pool or play poker. Basically, destress, and after a few days to a week of honest relaxation and rest, you should be back to normal.
If it persists it may be psychological, since you're worried and all. Doing the above activities should help. If it persists, see a shrink. I highly doubt your condition is physiological.
One thing to mention ... besides, women like men who are more controlled. So see this fall off in desire as a good thing to show your more mature side.