Cairn Terrier acts like something is after her all the time

My dog acts like something is after her all the time.  She is very nervous, gittery, and bites (licks) at her front legs (up high almost in her arm pit) just like she has fleas.  I cant find any signs of ticks or fleas.  She also runs and hides under things constantly just like someone or something is after her.  She hovers around me like she has a safe zone.  If i am touching her she seems to be fine.  Any help.......??????

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