Late Janurary 2016 I started using a new shampoo. It gave me all of these skinless bumps on my skin similar to chicken skin (I merely thought it was that). Around ten weeks after using it I decided that I wanted to have my smooth neck skin better and it wasn't worth compromising that for soft hair. As soon as I went off it these little bumps which I thought were chicken skin began to break out.
The photos show the progression. Image one was after I stopped using the shampoo, they look like closed comdones. My doctor prescribed me differin and ever since using differin for the past two weeks (using every three days) my neck acne has been going CRAZY!! I'm not even sure if this is acne or something else because it was caused by the shampoo.
This is the only thing I can think of: There was a chemical my skin reacted to in the shampoo and it clogged my neck pores up. Now that I've stopped using it all these little blocked pores need to open and the differin is speeding that up. Is this correct? All my derm told me was is that its acne but is my assumption correct? I never get acne on my neck, never ever, I'm a 16 year old girl and this is really affecting me. The last photo is the most recent, taken today. I was shocked at how bad it has gotten.
Any advice?
1) When I first stopped using the shampoo and noticed I was breaking out. 
2) Doesn't really show the bumps but this is when the actual pimples started coming. 
This shows the actual spots. I was alarmed to find that there were actually black heads. This made me think they were clogged pores and I decided to listen to my doctor and use the differin. Next images represent the next two weeks of using differn. 
Diferin week one. 
Differn week 2 - now!! 🙁
I had an extremely similar reaction when I started using Differin. I was using the 0.01% potency gel and it still caused me breakouts so severe that they were painful, I woke up every morning with more pimples and got to the point where I didn't want to leave the house.
I was prescribed the treatment by a GP when I started experiencing unusual breakouts around my jaw line and neck. Since I had never broken out in these areas before, and because they are the places my hair falls, I thought that I was having an adverse reaction to a dry shampoo or had contracted some kind of bacterial infection.
The GP gave me very little information, but after my skin started getting worse I read about Differin online, and became familiar with the term 'purging.' I read reviews of people saying that Differin had 'purged' their skin for 3-6 months, and then their skin had cleared up beautifully, because of these reviews I kept using Differin for over 6 weeks, even though my skin was getting progressively more irritated, more broken out, and I was beginning to experience severe hyperpigmentation and scarring.
I think it is possible that some people just react badly to retinoids, and I personally feel that the concept of 'purging' is largely a notion pushed by the beauty and pharmaceutical industry who want you to keep using their products no matter what. There is a difference between switching moisturisers, for example, and experiencing a few small whiteheads as your skin adjusts, and a full blown reaction.
I would strongly advise that you consider discontinuing the use of the Differin and waiting at least a month, using very basic skincare, to see if the reaction begins to subside.
Best of luck,
J