Yes, that is acctually to be expected. Any time you take a drug that replaces or enhances a natural brain chemical, your brain learns that it needs to produce less.
When you stop the medication, you are, at least temporarily, much worse off than before. All kinds of medications produce withdrawal symptoms.
This is also true. The SSRI medication is said to be non addictive, but coming off of the medication needs to be done slowly. The body adapts to the excess seretonin by dropping its own production a bit, and when taken off the medication the flood of depressing feelings is not uncommon, and can be devastating to some people. There are people who are helped by these drugs, but the drugs are over prescribed, and are dangerous to prescribe to those who don't need them. Sociopaths are treated with drugs which work against seretonin receptors. It is clear to me, that these drugs decrease inhibitions (thus the Social Dysfunction treatment claims). For alot of people these drugs supress the conscience, and thus we are enabling already disturbed people to do things they most likely wouldn't have done.