When choosing a location to perform a compass calibration, be sure to select a place that is not close to ferrous (iron based) metals. Most concrete is poured over rebar or wire mesh, therefore is not a good candidate. Also, stay clear of any electrical wiring close-by.
It could be that your compass has become magnetized. This can occur for any number of reasons, but can often be corrected without repairs or replacement of parts. There was a good discussion a few months ago talking about the various ways to degauss (demagnetize) the Spark. Here's a link:
Compass degaussing
Start reading at post #19.