I was flying under class bravo airspace at 1800 feet which was the legal altitude 1800 and below but my transponder is off by 200 feet so they thought I was 2000. when I landed and i had to talk to the tower. He did not say anything except be careful and turn your altimeter down next time and be careful. nothing else happened but am I OK? did the get my n number ? i never gave them my name or info.
You're probably fine. File a ASRS form all the same though.
The feds tend to focus on safety of flight issues or gross violations that indicate general incompetence. Some guy flying below a B shelf doesn't rise to the level of incompetence (because it's not). Worst case, you caused a resolution advisory for an airliner which, in the grand scheme, is not a huge deal (for you, legally) if you were following the rules (sounds like you were).
If you get a letter, call an aviation attorney. I doubt it will come to that though.
Keep on flying.