My best friend came and told me she was pregnant, I got all my maternity clothes for her and she decided what she wanted. Since flying an extra suitcase is so expensive we decided I would mail the clothes, I thought I should test before I sent them, since we hadn't actively tried to prevent pregnancy. I had the faintest line my husband made fun of me, well jokes on him...I'm due Dec. 16th.
The lines are a reaction of protein in your urine that attaches to a receptor in the test. If you're newly pregnant you don't have as much protein, if you can see the line, I'll go 99% your pregnant, if it makes you feel better, test again in a few days. Also, I used dollar tree $1 tests, I had in a previous pregnancy tested with one of those and then clear blue easy and EPT. I bought double packs of both and 2 dollar tree tests. I got a + with the dollar store and nothing on the other 2. I tested again in about 3 days, and got 2 +s and I think it was EPT that was still negative, so some test are more sensitive, but I'll only use $1 test from now on.
Good luck and congrats.