The ECG machine at the health centre may have been one of those automated models that attempts to analyse the tracing - they usually print out the "analysis" on the trace too.
The "analysis" is often completely innapropriate or just plain wrong (it's not very intelligent on workaday machines) - your GP may have taken it at face value, hence the alarm. The folks at the health centre might also have connected up the leads wrong (which gives very bizarre results).
If they haven't lost the trace, get someone who understands ECGs properly (not me) to look at it.
