Actual people who are labelled nerds, even by themselves, are human beings, individual and different from each other. They do not have uniform behavior.
The “nerd” stereotype is a caricature (i.e. exaggeration and simplification of a discriminatory idea grouping people together), and inaccurate. It also varies based on how exaggerated the caricature is. Cookieman’s joke answer above is an example.
Calling oneself a nerd all the time may tend to indicate a deprecatory self-image, and social discomfort. Many people who are uncomfortable in romantic social situations may tend not to express themselves much about it, if at all, and to be nervous, perhaps fearful, clumsy, and peculiar when they do.