How do I know if my thermometer is broken?

Andrea

Got a BBT of 97.58 for the 4th day in a row this morning. Took my temp seconds after with just a regular thermometer and got 98.1. Decided to see what they read when I got home from work just now and BB thermometer said 98.38. Used the regular thermometer again right after and got 98.0 and then did it one more time directly after and got 98.8. This was all within seconds of each other. Soooo... I'm not sure which one to trust 😩