That's stamina, not chakra. Of course she has low stamina since she is old, but one's chakra increases through training and other experiences, which we can be sure of Chiyo has more chakra than Zabuza. [She can't use them infinitely, they take a good amount of chakra and she only has a 2/5 for chakra according to the data books, which is even lower than Itachi's. If she even can set a trap for them again, I doubt she'd even have the chakra to do so
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], while her physical condition may bring her chakra levels down, we certainly know that her battle experience greatly contributes to her chakra.So basically, she can easily use these shields as much as she wants, especially since she used them three times, then continued to fight on for a long time without any signs that she was running out of chakra.