Losing hair color

Okay ladies I need some help. I dyed my hair from my natural medium brown color to black on sept 6. I’ve dyed it black from my natural color before but with box dye. The only difference is with the box dye I had color washing out like 3 times after washing it then I was good. Now with it done professionally I’ve washed it maybe 6 or 7 times since the 6th and I’m losing a TON of color. I texted my stylist and he said it was the shampoo probably. But I always use color safe sulfate free and I wash with cold water. I’m just so frustrated because I love my hair and obviously don’t want to lose it prematurely because I’m starting to notice right at my roots they are looking their natural color again 😣😣. Please help a girl out!!!!!!