When the sound waves that are produced by a ultrasound machine reflect there happens an increase into the frequency of the sounds. Why is that so? I think it should be the other way around because the blood is streaming and the distance between the reflected waves should be more, which should cause a lower frequency.
The Doppler effect can cause reflected sounds to be of a higher or a lower frequency; it depends on the velocity of the reflecting object. If an object is moving towards the source, then the sound reflected from that object will have a higher frequency than the original frequency; if the object is moving away from the source, then the reflected sound will have a lower frequency.
For a Doppler ultrasound probe placed near a blood vessel, there will be some blood cells that will be moving towards the probe: those that are "upstream" from the probe and therefore moving towards the probe. The reflected sound from these cells will be higher than the original frequency. On the other hand, the sound waves reflected from the blood cells "downstream" of the probe will be lower in frequency, since these blood cells are moving away from the probe.
Ultrasound machines capable of doing Doppler flow measurements can measure flow rates away and toward the ultrasound probe. Flow direction can be indicated by sound, or visually in different colours on the ultrasound image.