Yes. . Yes. Due to sensitivity from removing the actual bracket or sensitivity from the periodontal ligament.
Absolutely. You can take Ibuprofen exactly as directed on the package, and eat a soft high protein diet for a few days, until your teeth get used to being braces-free. Be sure to follow retainer instructions to the letter, look for reasons to put them in rather than excuses to take them out, if you want post-treatment stability.
Yes. Stay away from crunchy and sticky foods until your teeth get used to the new situation of not having your teeth stuck together and supporting each other.
Absolutely. Just like with pressure applied by braces causing discomfort, releasing that pressure will also be uncomfortable as teeth "settle." continue warm salt water rinses and analgesics that you used with braces.