Same man, I too get blocks when caught off guard, but I also get them when i build up tension for what's about to come. Its weird, I have realized its incredibly important to be calm in all situations, it can get harder, especially when there is constant anxiety due to stuttering. But, that is the only way you can ever deal with it. I am 24 yo male, I have struggled with stuttering all my life and have had countless experiences like yours. So, dont think about it too much, just try to remain calm and composed. It will get easier. Hope this helps!