Andrea, if you go to Swedish Roots and click on Swedish names, there's a general introduction to how Swedish surnames are formed.
-in is a pretty common suffix in later Swedish surnames, though that page doesn't mention it. I've read about this somewhere else but I can't recall where so I may be wrong in some details: In the 1700s when France had great influence on fashionable society, French-sounding -in and -?n surname endings were introduced. First they became popular with the upper classes, but by the late 1800s the French origins were forgotten and the now supposedly Swedish suffix had spread to all levels of society. At this time many people abandoned their patronymics (father's name + 's son or 's daughter, though in Swedish of course) and created new surnames for themselves, often using a place name as a root as Mats said above, and sticking on the -in or -?n suffix so it sounded like a proper surname. My own surname was taken in that way, as late as in the 1920s I think.
If you find out where your ancestor came from, you'll probably find the origin of the name as well.