Dear Megan -- 
Bipolar disorder can cause anxiety, that's quite clear (the link leads to an essay on that topic).  Could lithium be causing it?  I would doubt that, as I'm not sure I've seen it, or if so, only rarely.  On the other hand, one of my colleagues likes to say "medications are guilty until proven innocent".  So at some point you and your doctor might decide to take the lithium out just to answer that question.  However, especially if it's working in some respect, you'd definitely want to taper that lithium because we know that stopping it fast can induce symptom return in a lot of people, so we generally take up to 6 months to take it out completely.  You could probably figure out if lowering lithium was going to help, though, after the very first dose reduction, if you and your doctor agree on this "test".  Don't do this without her/him, though:  now that you're taking the medications just as prescribed, you have the great advantage of another observer who's watching how you look on the various medication arrangements and can detect changes you might miss (there's a form you can use to track this kind of thing, if you want to watch really closely and keep a record; it's hard to do every day but even if you miss half the time it's still very useful, especially when you make some changes and want to remember how things used to be).  

Dr. Phelps