Contacting references after starting job

Is it usual for a company to contact the references that you provided 3 weeks after starting the new job? I also get my first pay check in a couple of days. A bit strange, thought they would have checked after the offer stage and before the starting date. Anything to worry about?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official