I recently began stuttering pretty badly. For the last year or so I've been reading so much less than I used to (and I'm a pretty huge bibliophile). This does change for me as I cycle, but it is still pretty bad. About three years ago (when I was not medicated) I was almost unable to speak, I was always searching for the right word.
I guess words are hard for most people with bipolar... I think it is a combination of the racing thoughts that we can't verbally keep up with, the lack of ability to concentrate, and then the fixation on things - kind of OCD-like.
It is a pretty common problem, but if it is really bothering you, talk to your doctor about it. It might have to do with the new meds, since it never happened before and you just started taking them.

I hope it works out for you