Anyone else had this problem or know what might be causing it?
Which OS are you using as it sounds like the sticky key option is active for you, also, does it happen with any other game/application?
If Sticky keys are turned on and you're using Windows, pressing shift 5 times in a row will show its options, from there you can turn them off.
I'm unsure how to access these options on Mac, sorry.
But I hope that helps.