I'm feeling discouraged after getting tenure; should I quit? A few months ago, and seven years after I obtained my PhD, I finally got tenure.  Hence I have a permanent position in academia.

After a few months in this new position, I find myself thinking of quitting academia. I do not know if this is because of my new status but I am quite discouraged and not motivated to do research. Meanwhile, it seems that I now face even more teaching, more administration and more research pressure than before.

Have you passed through a similar phase, and if so could you describe your experience?  I wonder if all these thoughts are caused by the transition from being a postdoc/assistant professor to being an associate professor, but I do not know.
Many faculty get post-tenure blues. There are several causes:


Burnout: You've been running at full speed for almost 30 years (k-12; college; grad school; post-docs; tenure track). Your brain needs a rest. It's perfectly acceptable for you to take a break. Many faculty have post-tenure slumps. 
Survivor's guilt: Many of your grad peers didn't get jobs, you might have seen other faculty not make tenure. The tenuring process can seem capricious. You've just been through the sausage machine and you didn't like what you saw.
Imposter syndrome: Related to the above, you can't quite believe that you made it and you're doubting yourself. Were you really qualified to make it? Maybe there was a mistake in your file that they might catch? 
Fear of commitment: You look at the other tenured faculty and are deathly afraid that you're now committed to be with them for the next 30 years. Oh god, what have you done. Your dreams of leaving academia and opening a bookstore-cafe by the waters of Lake Michigan are maybe just that, dreams.
Fresh fish: At many places, newly tenured faculty are fresh fish for burdensome committee assignments. e.g.: No one really wants to be the MA grad advisor but it has to be filled by a senior faculty member, thus the 'new guy' gets it. Some senior colleagues may actually tell you that you "owe" them because they "gave" you tenure. Bollycocks. You earned your tenure. You owe no one. Just try to remember the 'just say no' skills that you had as junior faculty and wriggle out of any and all commitments until you can figure out which ones you can tolerate.
Peter Principle: The Peter Principle ("people rise to their level of incompetence") is often misunderstood. It's not that you are incompetent, it's that you've been competent in one domain but promoted to a higher domain than where you've shown competence. Associate and full professors often have to do much more administrative work and intrauniversity politics than assistant/untenured faculty. When you're tenured, work becomes less fun and more bureaucratic.  And you wondered why your senior faculty were all sourpusses. Key to your sanity here is to remain in domains that you're passionate about (teaching, basic research, etc.) and enter into new domains only on your own terms. For example, if the provost wants you to be chair of XYZ onerous committee, then you need $x research funds or to go to y conference or get z course relief (h/t to J and Captain Emacs). 
Paradise Syndrome also known as the dog who actually caught the car syndrome. You've been working so hard for so many years for something you finally achieved and suddenly you don't know what to do with it. You have to come up with new life goals and purpose for living (thx Pharap). 


The most important thing to remember is that they can't fire you without serious cause. Take a break, say no, piss some people off by not taking on work. Let your brain stretch. Go kayaking. Work on your bucket list. And then after that, try to remember what made you so passionate about your field.

Finally, if you really, really, really want to quit and you can't take sabbatical or get a grant to buy you out for a semester then: take an unpaid leave. The provost should be willing to work with you - if not, you can use FMLA or sick leave if you can get your doctor to agree you need time off. It's better than quitting as it leaves you with your Plan A when you recover.