Are there any fields which combine medicine and forensic science?

Yes. Coroners are actually required to combine a medical degree with advanced criminology education. I suggest minoring in criminology as an undergraduate while you slog through the pre-med curriculum. Look for a university in a city, like Chicago or Cincinnati.

I know 2 people who are pathologists (doctors that examine bodies to find out what caused their deaths). Pathologists are often called "the doctor's doctor" because they have so much knowledge about how the body works. The 2 people that I know enjoy their jobs. One of them is over 80 years old but continues to work as a doctor that reviews cases and testifies in court about them. I also met another pathologist who didn't enjoy the job because there wasn't a lot of interaction with other people.

Arrange to meet with a pathologist in your neighborhood so you can talk to him/her about their job and get an idea of what they do every day. They can also guide you on the education needed (medical school plus residency after school).

Good luck in your endeavors! You may be a real life CSI person in the future!