Why is Winston scared of his mother in 1984?

In 1984, Winston isn't scared of his mother. What we actually find (in Part Two, Chapter Seven) is that Winston has spent his adult life believing that he murdered his mother. The recollection of a key memory from his childhood, however, makes him realize that this assumption is false and that she, in fact, disappeared when he was just a boy.
That day in question relates to a chocolate ration that was issued by the Party. Instead of being content with his ration, Winston stole his sister's chocolate and then fled the house. As his mother called him back, Winston ignored her and kept on running. When he finally did return to the house sometime later, his mother and sister were gone, never to be seen again.
Winston's feelings towards his mother are, therefore, feelings of shame and regret, rather than anger.