An answer based on definitions:
In Jewish tradition a "concubine" (Hebrew: פִּילֶגֶשׁ; "pilegesh") is a specific type of "wife." This is a different concept from oriental concubinage of the Japanese of Chinese variety where a concubine received few if any legal protections or respect. For the Israelites, she would not be the primary wife, but had equal standing with her for the most part. According the Jewish Encyclopedia
She enjoyed the same rights in the house as the legitimate wife... The concubine commanded the same respect and inviolability as
the wife... The children of the concubine had equal rights with those of the legitimate wife... Jacob's sons by Bilhah and Zilpah were equal with his sons by Leah and Rachel.
In Patriarchal times, primary wives voluntarily shared their husband with a concubine because of the need to bear many children and thus fulfil the blessing to "be fruitful and multiply." In the time of the kings, however, this was no longer the case. Concubines were given to a husband by a wife anymore to increase the family's population, but became signs of luxury for wealthy and powerful men.
An answer based on Source Criticism:
Another explanation for the two terms, as applied to Bilhah, is suggested by source criticism. According to the Interpreter's Bible, iššâ is probably used by the E source, while pilegesh is used by the J source. The same thesis explains why one verse uses the name Jacob while the other calls the same person Israel. This is a simple solution to the question but will not satisfy readers who reject the documentary hypothesis of Genesis' sources.
Conclusion: There are two basic explanations: 1. A concubine is a type of wife, and the author (traditionally believed to be Moses) simply used iššâ in one verse and pilegesh in another. 2. Genesis is composed of several sources, one of which used the word for "wife" while another used "concubine."