The previous poster is correct; Grendel's mother comes for revenge. I want to add just a few interesting details.
Mother comes for revenge only after everyone has already celebrated Grendel's death at the hand of Beowulf, including Hrothgar, who showers Beowulf with gifts and says that he's ready to adopt him. (I think this last detail is what makes it possible for Beowulf to later assume the position of king of the Geats.) She's a much worse adversary than Grendel, of course, and the passages describing her underwater lair and Beowulf's battle with her (e.g. the blood bubbling up that is seen by Beowulf's men as a bad sign) are among the best in the entire work.
To me, Beowulf essentially shows here that he's not just fulfilling the letter of the agreement with Hrothgar. He had already killed Grendel and, by all rights, could have moved on with his rewards, leaving someone else to deal with the revenge-seeking mother.