I don't think any serious bruise
come within a overnight , that too without having pain or swelling, i think it might be due to some allergy
or some small insect bite, and i have never heard of any bruise in abdomen related to headache or dizziness
If you still feel uneasiness, you are kindly advised to get your lesion examined to nearby general physician, or rather send us the image of bruise.
hope my info helped you
Dr Hari Om Chandrakar