All you can be sure of is that they either have a pocket pair or a strong 9x hand and that they think you missed. A set is possible, but many other hands would play that way here as well.

With as many callers as you seem to be getting, you might think about adopting the method of making your raises bigger when limpers have already called (i.e. standard raise = to $4, but add .5 or 1 for every limper, so here you'd raise to $5 or $6). That should probably cut down on the number of callers you get, at least.