So I have a normal sexual life with my partner(we don't use condom anymore) and the other days I masturbated and when I ejaculated semen came with light brown liquid and I don't know what it causes it. I have no pain at all or not discomfort, the semen is white but comes with some light brown liquid. I do work sitting down all day.

The brown color is blood so it seems you have torn a small blood vessel in your penis. Don't masturbate or have sex for a few days, allow it to heal.