How does Phenylephrine cause goosebumps?

Phenylephrine works by stimulating the alpha adrenergic receptors in the body. Alpha adrenergic receptors stimulate muscles in blood vessels, causing them to contract, which results in vasoconstriction (blood vessels get smaller) - in the nose, this works as a decongestant because there is less fluid in the area as a result of the blood vessels being smaller (which means less puffiness, less leaking, etc.). These same receptors also control the tiny piloerector muscles in the skin, so phenylephrine can activate them as well. The piloerectors would have puffed out the fur on some distant, furry human ancestor to make them look more scary, or to increase the insulating layer of air when they got cold. But, since we don't have fur anymore, all we get are kind of lame goosebumps where the fur would have attached.

Phenylephrine Side Effect