The cause

Three hours had passed before Max was finally able to regenerate. Technically speaking, he had gone with one arm within that time frame and he felt weaker than ever.

 As soon as his HP was restored and he regenerated his left arm, he began heading to the room where Hillary had been kept for examination and treatment. Despite the words of the physician to not leave his room until he had taken enough rest, he ran off in search of her. The trigger to do so was his eagerness to find out what had gone wrong with her.

 Now that the doctor had explained what went wrong and had also seemed to do something to redeem her from the problem, he was glad to see the girl he knew once again.

 "Hillary?" He said in a calm voice.

The girl slowly turned her head to face him. At first, her expression read of how weak and sore she felt but after a few seconds, it twisted into one of great surprise.