The correct answer is "Brutus has been acting strangely because he has a lot on his mind that is troubling him".
We can interpret that in this small excerpt from "Julius Caesar", Brutus is offering a kind of apology to Cassius. He explains that he's conscious of his erratic behavior, but it's not his intention to cause any trouble. The fact of the matter is that Brutus has been dealing with a lot of issues on his own, and his mind is quite obstructed. These ideas can be easily inferred from this pair of quotes: "Vexed I am of late with passions of some difference. Conceptions only proper to myself, which give some soil, perhaps, to my behaviours", "Nor construe any further my neglect than that poor Brutus, with himself at war, forgets the shows of love to other men".
Hope this helps!