http://www.webmd.com/sexual-conditions/understanding-male-sexual-problems-basics
The above website will give you general information (medical) on sexual disorder.
I suffer from hopelessness, bad mood and irritability after ejaculation, be it sex or masturbation. This starts about one hour after the ejaculation. I have no guilty feeling, I have a very loving partner and I am satisfied with my sex life. I also feel very uninterested about sex during this period. The video that seemed very interesting and helped me to masturbate now grosses me out. The body of my partner, which seemed like a "wonderland" during sex, now seems no more interesting than a man's body. The normal feelings usually get restored after 20-24 hours and then there is a feeling of hypersexuality for another 24 hours. I don't know the reason, but I am sure it has got something to do with Testosterone and some Neurotransmitter levels.
Yes, it is a normal reaction when we feel tired/sleepy/relaxed after masturbation. It can be caused (among other things) by substances released into brain at the time (or short time after) orgasm. Orgasm is accompanied by the release of substances which are responsible for those feelings.
Masturbation and sex - physically it is of course different, I mean different kinds of stimulation. Some men have also different quality and quantity of their ejaculation in case of masturbation and having sex with a partner. But the basic mechanisms/reactions are the same (erection, ejaculation etc)
Mentally - it depends on individual people, but I would say that there are big differences. In my case, for example, masturbation was often followed by sadness, not because of guilt or prejudices against masturbation, but because of the sadness that I was alone and without a partner when I desperately wanted to have a partner. Having sex with a partner can be related to intense emotional reactions and bonds between two people, which is not the case with solitary solo activities. However, many people continue with masturbation also when they have regular sex life with their partners. So masturbation has its own benefits, too. Needless to say, sex with a partner can be also accompanied by many negative emotions - e.g. when you are rejected by a partner, or performance anxiety, if you cannot feel relaxed and enjoy sexual pleasure in the presence of a partner - these things are missing in case of masturbation, of course.
Last but not least, some couples can enjoy also watching each other masturbate - it is a big turn on for many people. Some couples can enjoy for example the scenario of "You can watch me how I play with myself but you are not allowed to touch me". Needless to say, it can be extremely arousing if you watch your girlfriend masturbating and having intense orgasms right in front of you, so masturbation can be implemented also in the sex life of couples.