it-ogo explained it really well!

"ПсихологИня " is a colloquial form of the feminine noun "психОлог".
Sounds similar to the masc - fem. related nouns:
-- Бог" - богИня,
-- герой - героИня,
-- раб -- рабЫня.

"общага" is the соlloqial abbreviation of "общежитие" (dormitory).