Bug bite? Yesterday I noticed red swollen circle about 3cm on my leg. Today it's about 15cm & has a white mark/dent near the middle & feels very warm.

Infection. Sounds like a possible infection. I would see your doctor about it.
Get it examined. The fact that it is larger is of concern. That it is warm even more so. It may be a spider bite. This can be serious if is the brown recluse spider. You must get it looked at right away.
Sounds infected. Keep area nice and clean with warm water and soap. You need to be seen by a doctor as soon as possible and likely get either some antibiotics or some type of drainage procedure. But you need to get this done now.
Home remedy. Apply an ice pack over the slewing. Use NSAID for pain and inflammation management. Mix water with a baking soda and apply over the area or Benadryl (diphenhydramine) OTC to reduce itching. FYI for the future, the best way to "treat" bug bites is to avoid being bitten. If concerned see your PCP.