I have a male chihuahua who is about 8 years old who goes outside to urinate and have a bowel movement; however, he sometimes comes inside and then proceeds to rub his behind on the floor or on my bed.  What could be causing this?  Should I worry enough to have him checked at the vet?  Thank you.

Could be a clog in the anal glands.  This is common behavior when this occurs.  You can go to the Vet or a groomer & ask for the glands to be drained (pampered pups have this done when they visit the groomer).  See if this does the trick. 

I also agree, he could need his anal glands expressed. This is most likely the cause. Other more serious conditions could be Rectal Prolapse. You should just take him to your regular vet to be certain.