The main reason for blood appearing 'thin' in the syringe is anaemia; failure to clot properly has many causes.
Tests needed to clarify this would be a FBC (full blood count) and clotting screen for a start.
If it was just a test for diabetes these tests would have been unlikely to have been done.
Of course this was all very subjective, and the nurse might have been mistaken.
No harm in ringing your GP to discuss it, as he/she may be able to order further tests without necessarily making an appointment, and also you could check whether a FBC was done.