Some are flat and itch like the dickens. If it was a spider or a mosquito have it checked, because of the infections that come with bites. When a spider or mosquito bites, it leaves a secretion under the which is why they itch, but mosquito bites will itch more.
The spider's mark may swell into a bump and stay hard for a while, but it is still advisable to go to a specialist doctor to have it checked out.
You can find many more similar questions and much more information about bites at our specialized insect bite page HERE: http://insect-bite.blurtit.com/