Victor Frankenstein has a surprise visit by his friend Henry Clerval the day after the monster came to life. Frankenstein ran into Henry in the town and as the two walk back to Frankenstein's place he grows more and more nervous about Henry seeing the monster. He narrates:
"My hand was already on the lock of the door before I recollected myself. I then paused; and a cold shivering came over me. I threw the door open..."
I think Victor so worked himself up in fear about Henry seeing the terrible creation that Victor made, that Victor made himself sick. As the two continue through the house, Victor imagines he sees the monster, but the monster isn't there. Henry assumes the worst of Victor and begins to nurse him.
Although Clerval had finally convinced his father to let him study the great books at this university, Clerval ended up spending this semester tending to the needs of Victor. This illness of Victor's was very much about the monster, but Henry kept assuming Victor had an overactive imagination. Clerval finally gets Victor to read some letters from home.