It certainly is. It's not weird at all.

Have a look back over your DBT material. The principles remain the same even if your anxiety issues have changed.

You are changing meds and are naturally worried about potential side effects. If you read all the accompanying literature on lexapro you will fill your head with panic and "what if" scenarios. You need to give the lexapro a chance by trying to manage your anxiety more effectively. Keep up with the walking and keep occupied with stuff which isn't entirely MH related.