Hello James,
Thank you very much for your post and welcome to our forum.
I do not believe that what happened has put you at any significant risk of an STI.
The only infections that you might have possibly contracted, if anything at all, from the anal rimming would have been anal Chlamydia or Gonorrhoea; you can have ana anal swab if you want to, even though it is not absoultely necessary if you do not have any anal symptoms. Other that that, no need to be concerned at all whatsoever.
Best wishes,
José