Is it really so important how others view this "hypothetical" scenario? And, if so, why?
The reason I ask is because, as an addict, I can look for answers already having made up my mind about how I see things. Part of it is that I hope others will agree with my conclusions or cosign my crap. Another part is that I could be looking for an excuse to disqualify myself (i.e., "F%$K NA!! What do they know anyway?")
With that said, here's my opinion: I do not believe it was a relapse because your intentions were not to get high. But I also believe that you were walking on shaky ground by taking medication that [1] wasn't prescribed for you, [2] may have been similar to your drug of choice, and [3] could've set off a chain reaction that led to you actually getting high. There's plenty of non-narcotic pain relievers on the market that could've been obtained legitimately without risking your recovery.
I'm with Phinny...what did your sponsor say about this? Did you discuss it with your NA group?