Ammonites with uneven coils like these are called heteromorph ammonites. Ammonites from the Lower Chalk did all sorts of strange things. Some are partially uncoiled and others became bizarrely decorated with tubercles and spines. This one, called Idiohamites, is no exception and is in fact an extreme example as it has almost completely uncoiled.
Find out more about ammonites on the molluscs pages.