I'm pretty sure your period can't just change like that out of the blue, the only thing that can really change your periods, are pregnancy and Birth control!..
Anyways, if you tested in the afternoon your urine was probably too diluted to work, so take another test FIRST THING IN THE MORNING. Also dark brown blood is OLD blood... so that could be your body getting rid of the rest from your period! Are your periods Regular or Irregular?
And have you been having Unprotected sex? Cause if you have that could lead to pregnancy!!
If your urine test comes out negative, and you feel pregnant or miss your next period, go and ask your doctor to do a HCG blood test on you.
x
maybe your cycle is changing