there are a few things that can contribute to your light period. I don't know your age though.
Do you have a history of ovarian cysts?
Are you on birth control pills? because they can cause light periods, and so can ovarin cysts which make you miss a cycle which would make the period very light.
if you do research on how a womans reproductive system works, it's not all that simple and it's a fine tune thing that occurs each month. If something so minute goes wrong, such as having an ovarian cyst, it gets capsulated inside the ovary and can't come down the fallopian tube and then all the other things can't happen as it does smoothly each month in order for glands to work to tell the uterus to build up and then slough off which hence means having your period.
I think you would find it absolutely fascinating to see how it all takes place each month. when you have the time, do some research and you'll see just how wrong things can go to delay the period and only cause spotting and by the way, brown blood means it's old blood.
when I had chronic problems with my ovarian cysts, which started in my twenties and only got worst well into my late mid 40's, and I wound up having to get a hysterectomy because of all the problems it gave me.
I would advise you that if it bothers you where you're not getting any rest over it, to call your gynocologist and make an apptn. for a pelvic exam.
IF you think you're pregnant, they have tests now that can detect pregnancy within one day of being late.
If you are on any form of hormonal birth control then this is probably the culprit. If you have recently started oral contraceptives (ie. within the last 6 months) then I would definitely suspect this first and foremost.
If you are not on any form of hormonal treatment then you need to look at any medication that you are taking, such as for thyroid, for example. This may definitely have an effect on your period.
Other than oral contraceptives and any other medications, some other things can have an effect on your menstrual cycle. Being underweight will lighten and perhaps even stop your period altogether. Having a condition called PCOS were you have a necklace like pattern of small cysts on your ovaries (although this is not always a symptom) may cause irregular periods. Sometimes even your emotional state of mind may have an effect on your period. Obviously, being pregnant would be the obvious cause but you said you have already considered this and tested for it.
If you are extremely concerned over this, why not make an appointment with your doctor or gynecologist, just to check it out. They will probably perform another pregnancy test and then consider your particular case in order to determine what this might be (if anything).