I have kept mice and rats in the same room with my snake and my monitor and I have never been bitten by either. I kept rats in the same room with an African rock python as well.. who can be notoriously agressive and was never struck at by her. So, to say that, that is the reason, is inconclusive at best... She could be feeling threatened.. How long have you had her? Some boa's..can be nippy and nervous as youngsters.. they mellow out eventually.
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Lucien
1.0 Columbian Redtail Boa (BCI)
3.1 Leopard geckos (2 Blizzard and 2 het Blizzard)
0.1 Savannah Monitor
13 rats
12 Gerbils
2 Dogs
3 cats
1 Albino Corey (fish)