Why does my semen not come out forcefully?

I am a 25 years old man who has been masturbating from the last 10 years. From the last few weeks, I have been observing that after masturbating, at the time of ejaculation, I feel the force with which the semen comes out is not much. It does not come out forcefully from the penis and is less in quantity. Also, I feel the semen moves backwards into the penis. What is the reason for this?