This may be part of the reason, but something that a lot of quitters also don't know is that nicotine also affects blood sugar levels and when you quit, it can cause blood sugars to become untable for the fist 2 or 3 days until the nicotine has completely cleared your system (another reason to avoid nicotine replacements as they keep the nicotine in your system longer and therefore contribute the the blood sugar instability for longer).
Unstable blood sugar levels in the first few days of a quit can trigger sweet cravings, but after the first 72 hours, it's not the lack of nicotine causing your cravings.
As for the serotonin connection, that is usually back to pre-smoking function within 2 weeks of being nicotine free.