Hi sammie! Like has been said, false negatives do occur. Also, your pregnacy is calculated from the first day of your last period, so that would make you 5 weeks by my reckoning. Another thing, it may be is that you have low HCG which is the hormone that you produce when pregnant and what the test picks up on to give positive result. I would advise taking another test but doing it first thing in the morning on your first wee of the day as this is when the level is highest.
You also have to consider that you may not be pregnant at all and there is another reason for your missed period.
I really hope it all goes the way you want it to.
Keep us posted, we are all here to help if we can. daffi x