Amphetamines stimulate your CNS/sympathetic nervous system and basically trigger your "fight or flight" response. The point of the fight or flight response is (drumroll).... to best prepare your body to fight or escape and does this by stimulating or channels resources, blood, hormones and other bodily "stuff" to the areas that would best allow you to do this. This includes a lot of things, and not all of it is relevant to your comments, but some of the body's preparation to fight or fly are to: stop digestion, increase heart rate and BP which increases the amount of blood that is pumped to various organs, dump more glucose into your bloodstream so it can be shuttled to the places it's needed, waking you up and giving you energy and basically making the body unlikely to have sex or discourages you from sex. Why? It's not evolutionarily beneficial to attempt to reproduce when a Mastadon is attempting to make you its dinner-- you have more pressing concerns!
To this end, blood is channeled to your CNS (Central Nervous System) or the most important organs that need to function optimally at this time. This includes your brain, your lungs (preparing you to run for your life), possibly your liver/pancreas, increasing the amount of glucose in your bloodstream (you're going to need energy in that long battle or sprint!) and other high-priority organs. It also channels blood 'AWAY from your periphery (ever get cold hands or feet despite being warm during stimulant use?) and away from your DICK not to mention other bodily responses that discourage optimal sexual performance (hormonal modulation that discourages sex) as well as your increase in blood pressure. People with chronic high blood pressure have a hard time getting and keeping it up and this is one of the conditions you hear discussed by the rugged dude in the Viagra commercial
So to briefly answer-- it's a common issue. Lol
You alluded to a bad comedown and not being able to sleep.... IMHO, this is where a little help from our Benzo friends can be VERY useful, making the process way more pleasant, but I am not sure what you can get your hands on.
Sorry for the long answer to the short question, but I figured I'd give a little bit of info behind the why of how your body behaves while under the influence of this substance. Best of luck to you and be safe! :D
-SpinningFree