Postpartum Hair Loss

How long after pregnancy can postpartum shedding continue for? My toddler is 13 months old and I'm still losing chunks of hair, especially when I wash it ( I wash it every second day. Can't go any longer than that, it gets too greasy). The drain fills up with hair every time I wash it. My hair is getting thinner and thinner, my daughter even commented that I have a bald spot at the back of my head! My hair gets everywhere, constantly. I had blood tests done when my son was about 2 weeks old, all came back normal including thyroid. I also spoke to my dr a few months ago who said it was likely still postpartum shedding and my previous bloods were normal so its nothing to worry about and it will grow back. It's not letting up though, I'm so worried I'm going to end up bald. Any tips? Special shampoo recommendations, etc. I'm already taking collagen supplements.
Thanks.