It is very normal at this age to start testing the truth. I wouldn't call them out and out lies, but rearranging the truth to suit themselves. It can be fixed though. All kids I think go through this.
I went through with my daughter who is 7. From here on out, double check everything she tells you in front of her, follow it up with "I hate doing this, however you have lied so many times I do not know how to believe you anymore". Punishment for lying in my house is immediate going to bed, no matter what time it is if it is a big lie. No exceptions. I have been very firm about lying and told my kids lying keeps anyone from every thinking you are able to be trusted. I told them when you lie you are headed down a path of self destruction and I will not tolerate liars. May sound harsh but growing up that was one thing we got in a lot of trouble for.
For those six months I could tell when she was lying, I would look her in the eyes and say "now, is this the story you are staying with? I am giving you one chance right now to be honest, if you lie then the punishment will be severe, so now, do you want to change what you just said with the truth?"...if she proceeds to lie, I find proof of her lie and (not always just assume). A breakthrough came when she developed a conscience during these routines of ours and would fess up immediately or start telling me something like "you know, I am not sure if this is true but..."..then it led to "I know I am going to get into trouble but I want to be honest"...so it is a gradual process and you have to stand firm, consistent and let her EARN your trust back.
It is a rule here too that they may get into trouble for telling me the truth if it is something bad, but lying will make it way worse!
I also told them (like we all know) that moms ALWAYS find a way of finding out the truth so to avoid more punishment it is best to be telling me the truth.