I think the new shampoo I switched to doesn't agree with me, because I've gotten pimples on my hairline and the back of my neck.

About a week ago, I switched to a shampoo recommended by my hairdresser because I loved the scent so much. However, starting the day before yesterday, a lot of tiny, itchy pimples began popping up along my hairline, behind my ears, and on the nape of my neck. I haven't changed any of my facial skincare products, so I've been wondering what the cause might be. I suspect the shampoo ingredients didn't agree with my scalp, or perhaps it didn't rinse out completely, causing irritation. It feels like such a waste to use it, but I'm planning to go back to my original mild shampoo starting today. Has anyone else had an experience like this after switching shampoos?