I have two parallel black lanes in my nail, for a week now ( there is a photo ). I don't think I have hit somewhere, neither I feel pain. I would like to know what can it be and if I should take some tests. The picture is not very good, if you need a better picture, let me know.

Some facts that may be related...
In general, I have dermatological issues in my head, have been told it's caused by the stress, given some medication but it comes and leaves.

Since last year, perhaps a bit longer, when I wake up, I always have in my mouth some white liquid, which I try to spill as soon as possible, but I have a feeling like never I can spill everything and I want to drink water.

My blood test are general good, in the limits, last year I had an increase in some indexes about the liver, so I stopped drinking for three months and it was back in normal levels again. Although, I was only drinking a bit more than the normal.

I am slim, I don't exercise and have a quite healthy diet, with too many sweets in some cases.

If you need to know anything else let me know.

Thank you for your answer in advance.