So I've searched around and can't find any information on this weird affect.

Back in my ecstasy days I would frequently get really weird goosebump type things on the inside of my forearm while rolling.
These bumps looked just like goosebumps from the chills but acted very strange.
They would only appear on a small part of my forearm and they would rapidly appear and change shape, then finally disappear after a minute or two of morphing shapes.

Considering I was taking ecstasy not pure MDMA I'm not quite sure what chemical was causing this reaction.
The only thing I can think of is that it was a weird reaction of overstimulated nerves, the thing is it only happened in one small area on my body.
The bumps also felt very weird and they were not a hallucination because I have shown them to my friends who confirmed them.

Does anyone know what was going on with my body and what chemical was causing this?

Remember that MDMA / ecstacy / stimulants will mess with your body temperature, and your body may well be struggling to keep a handle on things. I remember nights where I would eat 5-6 pills and end up with heat rash on the back of my neck, complete with bumpy/ blotchy bits. I tend not to have these irritating side effects with clean MDMA though...

I can see where you're coming from but these were definitely not heat rash.
It was not itchy or red just bumps that would move and I could sit there and watch them move around on my arm for about 5 minutes or so.
Like I said, the best thing I can chalk it up to is random firing of the nerves in my arm. Though I'm not sure why it was only in one small area, the same area every time.
I can't believe that I would be the only one to experience this, although I've never seen it happen on anyone else.