Hi there and welcome to the forum! Well, it is always hard to say as periods can just be abnormal at any given time for a woman. Things like stress, weight gain or loss, or a blip in hormones can cause a change in one's usual period.
Could there be any way you were pregnant? Chemical pregnancies/miscarriages happen in which someone did fertalize an egg but it happens very quickly that the pregnancy doesn't take. In times in which early pregnancy tests weren't available, women never even knew that they were pregnant for that couple two or three days. The faint line indicates you had some of the pregnancy hormone in your blood stream. While false negatives are common, false positives are less so.
So, I'd wait to see what kind of period you have this month. If it is late, take another pregnancy test.
If it continues to happen, you may want to check with your doctor to discuss what might be going on with your hormones. good luck dear