It's probably not a tick bite, ticks latch on and stay around until you pull them out.
If it's getting smaller it's probably getting better. It sounds like you had an allergic reaction to it to start with (like an aggravated mosquito/horsefly bite) and then your body dealt with it and it's getting better.
It sounds like it's nothing. If it starts hurting or a bruise or patterned ring develops it could be something serious so keep an eye on it if you are worried about it.