I seldom trust docs completely anymore, but when I was on Lexapro, I was suggested to go on a muscle relaxer, I think the same one you're talking about, and the doc said it wouldn't interfere with Lexapro. Don't remember what the situation was anymore, and I didn't take the muscle relaxer, but just one person's experience.
As long as you told your doctor you were taking Lexapro, he/she would not have put you on something contraindicated. Call your local pharmacist and ask them if there is any interactions with these meds. (There won't be) So relax, you are not harming yourself except by stressing out.
I can't see any reason a muscle relaxant would trigger anxiety, but we are all incredibly different in how we react to medications. You need to speak with your doctor about the increase in anxiety. If the pain is really bad, that could possibly set off some anxiety.....you could ask the pharmacist that as well.
You'll be fine.
Peace
Greenlydia