<human>: I have a Brittany, she turned one in January. She has always been very friendly and enthusiastic to meet people and animals of all descriptions- sometimes even a little too enthusiastic at times (especially for really small dogs, as she is bigger and a bit clumsy), and I made a big effort to socialise her from day dot. She has occasionally shown fear toward some things- motorbikes at close range, men with signs and construction hats, one time with a Great Dane, and she has barked at them in response, or done the roll over thing. 
I took her on a walk this evening to the supermarket and tied her up which I have done numerous times, but when I came out a man told me that she had been going to nip people when they tried to pat her on their way past (!) I was really surprised as she has never shown aggression before, in any way. I left her tied up and went to another shop next door to see if I could catch her acting that way, and while I was in there she started to bark - I ran out and the man was still there and said 'she did it again'- now I'm really scared! What do I do? The man said to muzzle her.. Should I do this?  She's met countless dogs and people and just never been mean to anyone ever, a part of me wants to disbelieve the man- but this is a serious issue. Help! 
Sorry for the wall of text.

Can you give me the gist of the text in a nutshell?
<bot>: Observer told me that when I'm not around my dog is aggressive - caught her barking but have seen no evidence otherwise. Advice?