Hi Cassandra,
Like you, I'm extremely frustrated by the lies my addicted sister tells me... and often, she seems to lie for no apparent reason! What I've been advised to do is ignore what she says and focus on what she does. It's not worth arguing over whether things are or are not true, particularly things that neither of us can really conclusively prove or disprove. Instead, when I think she's lying to me, I just shrug and say, okay, and change the subject. It's not worth my time or aggravation to dispute it.
But to answer your question, yes, I think the lying has everything to do with the addiction particularly if your ex wasn't dishonest before the addiction. And it seems to take time for that mindset to change even during the beginning stages of recovery. At least that's what I've been told... my sister hasn't actually had any real clean time for me to tell if this is true!
When I think she's lying I just remind myself that I'm waiting for her to DO the right thing. What she says is unimportant to me.