Eventhough all factors being constant, the endometrial thickness growth vaires few times significantly. The endometial tissue is the one which comes out with blood each cycle.
If endometrial growth is decreased in a cycle, the spotting would be less like brown discharge for you. this endometrial thickness variation due to minor hormonal
variations is normal and not avoidable. But this is temporary.
Dont worry about it. You will be fine in next cycle.
Apart from this , let me know if you are suspecting pregnancy
so that you can perform a pregnancy test. If no pregnancy is suspected there is no onther reason.