Why is Shylock very doubtful about Antonio regarding bond surety in Act 1, Scene 3 of The Merchant of Venice?

Shylock tells Bassanio that he is concerned about lending the
3,000 ducats to Antonio because he knows that Antonio has his investments tied up in all the
ships that he has sent abroad. While Antonio originally told his friends in Scene 1 that he was
not depressed because he had more money than what was invested, he later tells Bassanio that he
cannot afford to lend him money outright but that he will try to use his credit to get money for
Bassanio (it is unclear whether this is an inconsistency on Shakespeare's part or if Antonio is
not being upfront with Salarino and Solanio). Nonetheless, Shylock seems to be correct in
assuming that Antonio might be stretched too thin by borrowing 3,000 ducats when he is so heavily
invested in risky shipping at the moment.