What Does Itchy Rash On Both The Arms Indicate?

hi i just woke up (5 am) due to an itchy rash that has emerged on the back of both my arms. it appeared strongest & first on my right arm, & then quickly developed & began to spread from the back of my left arm. the rash on my right arm is more clustered while the rash on my left arm is more spattered. the bumps are small & pimple-like & searingly itchy & hot. I took a melatonin pill that looked weird (crumbling)- was there something wrong with it? is this my body flushing it out? or is it something else entirely?