Could it be anxiety? Do you notice this all the time or just in specific situations? Is it like a mental block or more of a stutter? The words you have trouble with, are they the same ones all the time or is it just words in general?
You should talk to your doctor about this. It could be part of your schizophrenia, but it could also be some side-effect of your medication (you are taking medication, right?). Give the doctor a call and ask if you should come in.