Why does jonathan have a nervous fit ?

Mina and Jonathan are in London when Jonathan sees the Count. Suddenly he seems to remember something. He is horrified, saying that the Count is here and is now grown young, but he is so upset that he passes out and on waking can remember nothing. Disturbed by these bouts of forgetfulness, Mina resolves to open Jonathan's diary and read it for his own sake. But that night, they arrive home to find a telegram informing them of the deaths of Lucy and Mrs. Westenra.