Sounds like she's a very protective mother, not wanting other lambs near her that may steal from her milk, and not wanting her lamb to be near other ewes that might hurt it.
Watch and see, if she's happy with her lamb, it's just a matter of time until she'll be comfortable back in the flock and you'll see her settle down. Sounds like it's perfectly natural and you're doing everything right to introduce her gently back to the flock.