I've had that happen on 2 cycles with cheapie tests. Both times I'm pretty convinced it was a sign of an HCG level that was just too low to show up properly yet! Some people told me they might be defective, but honestly, I'd used tests from the same lot several times, looked at them after the time limit and never seen a peep of a line at all, let alone one with color! The 2 cycles it happened on I started spotting like the next day (BBT had fallen etc) so I'm pretty sure I had chemical pregnancies due to my low progesterone.
I think this is a GREAT sign for you!!
This cycle, I had the line show up 5 minutes after the time limit... next day, it showed up within the time limit (though I'm doubling pretty fast so it may take 2 days until it shows up darker for you).,
I'm so excited, I think this is REALLY good news!!