You can get bruises from injections if you break one of the tiny capillary blood vessels, its nothing to worry about and the bruising usually disappears after a few days, just remember to rotate your injection sites.
It is quite normal to experience bruising, nothing to worry about. Make sure you inject in different sites each time. It is quite common for me to get bruises but I also make sure I use a big area in my injection site and move it at least an inch each injection. The bruises clear up after a couple of days!