Can microchips in your body be used to track you?

The short answer: No. 

The long answer: The microchips you're familiar with cannot have a range long enough to track someone. To be close enough to be in range, you'd be in physical up-close range of a few feet. Unless you have noticed strangers invading your personal space to get within less than 3 feet of you and scanning a particular part of your body, I wouldn't worry about it. 

Let's talk about those chips, except now since it's in a body; implants. Typically they are NFC or RFID frequencies. I'd like to reference you to a hobbyist manufacturer Dangerous Things who have summed up nicely why one would want to use these frequencies in body. 

The chips that are used in the human body use the frequencies because they are short range, and do not require any power to be read. The reader which scans the chip powers the chip enough to read it, and it does not require any power itself or any charging. The antennas in the chips cannot send or receive signal unless they are receiving the power. Thus, you'd have to have your hand on or within inches of a reader for the chip to function at all. 

Common applications of RFID cards are access cards for buildings and hotels, which are the same type used in the implants explained above. These are either 143.2kHz or 13.56MHz and have a range of 4 inches to 3 feet respectively. These ranges are if the implant is not interferred with in anyway, which is not the case once it's in the human body. Frequently for the implants to scan, they need to be within inches. 

If you're looking for RFID that can track people, you're looking for Active RFID, which is 433 MHz and can be read over 800 meters away. There's also 5.7GHz and 2.4 GHz RFID frequencies, but these only get about 20 meters away. In order for these to work, they must be actively powered. The antennas in the tag need enough power to broadcast that far, and thus need a built in battery pack in order to operate. They cannot be placed in the human body and are often too large to be placed on someone without noticing. However, they are used to track consenting people; patients in a health care facilities. These tags are badges that can be worn on lanyards, and easily removed. They are temporary objects, like a key, and can be dropped at anytime. 

Even if for some reason you believe you have microchips inside of you placed there without your consent, the ability for those to be used without you noticing is incredibly small. Critically address the usability of the conspiracy of being microchipped against your will. Some people choice to integrate technology into their bodies, and just because it is misunderstood does not make it evil or a complex theory of domination - it's just someone's door key.