Refilling an airforce bottle

Hi everyone! Got out and did some shooting today, and emptied the bottle to 1500 psi. I went in to fill the bottle and it started whistling. 😐 The more I opened the valve the louder the whistle was. 😯 I just filled the bottle as slow as I could, and disconnected it. The base of the neck on the airforce bottle wasn’t hot, but it was warmer than the rest of the bottle. Shot the air out of that bottle, then refilled the bottle again. this time it was normal. 😕
What’s going on? Has this happened to anyone else?
Oh, I have an airhog tank 88 cuft (i think) if that matters.