Test invalid?

Alexis

So I had a super light period and have been sick ever since... for about 4 days I lightly bled, had no clotting or cramps. I've had a TERRIBLE headache and fever for 4 days or so... went to the dr today and they tested for flu but it was negative. Just wanted to double check this afternoon. Took a test and the line instantly popped up, but then about 5 mins later it disappeared. Is it invalid?