I can't seem to say hello whenever a customer walks into the store?

I work at a grocery store and this started happening about 2 months ago. Whenever a customer walks into the store, I tried to greet them. But when I tried saying the word "hello" for some reason, my voice and diaphragm suddenly locks up. When that happens, I sound like I am panting for breath. I thought... show more I work at a grocery store and this started happening about 2 months ago. Whenever a customer walks into the store, I tried to greet them. But when I tried saying the word "hello" for some reason, my voice and diaphragm suddenly locks up. When that happens, I sound like I am panting for breath. I thought as this a some anxiety or nervousness. any idea what the problem might be?
Update: Thank you for the quick answers. I have been working as a cashier for 6 years and it was a sudden effect. I will just stick with saying "hi, how are you" for the moment until I can overcome this problem.
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