Going to dermatologist for hair loss I'm only 19, should I tell her I took plan B twice this year? (photos)

Yes, I would disclose all recent medications. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

It’s always important to tell your physician all the information including the medicine that you have recently taken or are currently taking, otherwise they won't have all the facts to better serve you.

When it comes to determining the cause of hair loss, every piece of information is potentially relevant. Information relating to Plan B is important as the hormone contained in the medication (1.5 mg levonorgestrel) could cause shedding in a smaller number of users - as can any sudden hormone change.
But all females with perceived hair thinning require blood tests.  If your hair does not improve be sure to see your dermatologist for repeat evaluation and consideration of ordering blood tests. Low iron levels are sometimes relevant to hair loss in young women as are anemias (low hemoglobin), thyroid issues. Other causes including hair styling practices, stress, diet, weight loss and general scalp health need to be reviewed.