Sirius is notorious for creating false images in eyepieces. To confirm a result you really must know what 10 arc seconds is in your eyepiece.
have a look at the this page:
http://www.astropix.com/HTML/B_WINTER/TRAPEZ.HTM the diagram of the trapezium shows the distances between stars - so we are talking the distance between a and b.
Then make absolutely sure it is in the correct position (90 degrees east of north)
If it isn't very hard indeed, it isn't the pup!