It seems to be a diminutive of
Aleksandra (and
Olga), made of the most popular diminutive "
Ola" and suffix "-encja". There are also forms like this for other names, for example "
Katarzyna ->
Kasia -> Kasiencja", "
Joanna ->
Asia -> Asiencja", "
Małgorzata -> Gosia -> Gosiencja".