Compass spinning like a top.

I was wondering if anyone out there could explain something to me. I'm in security sales and sometimes I'll spend the whole day riding around in the country looking for new construction. Sometimes it's easy to get lost so I bought a $12.00 compass which once calibrated seems to be accurate. If I know I'm heading West it shows that. If I turn right it shows I'm heading North. One day last summer I just happened to look down at the dash and the compass was spinning like a top. I've been through the same area many times and nothing. Are there really things like vortexes or could I have driven through a magnetic field. What do you think would have caused that. There were no overhead power lines and I immediately looked up to see if it was a ufo and of course there wasn't one. Maybe I'm just making to big a deal out of it and the people I've told think it was weird but don't have an answer and I would really like to know. Thanks.

[edit on 9-12-2007 by Mutantalien1947]