Yeah, this happens, although it happens more when you're old than when you're young. It can be because you like it fast and hard and she likes it slow, and if it stopped feeling good to you or even if it wasn't what you expected, it can happen. But sex is different pretty much every time you have it, so just enjoy it. Changing speeds is good to learn, by the way, as it will help you last longer and enjoy it longer. Every change can delay orgasm, including changing positions. Good things to learn, and you are learning them. Sex is like anything else, it takes time to learn how to do it. And the above is right, having a partner who communicated with you about what she liked is very rare in life so treasure it. Peace.
Yep, that's normal. When you change speed, you might become a little less aroused, and the stimulus of it all changes, so your erection might change.
It might not happen every time, or if you start slower and pick up speed later, or it might - everyone is different, and it's all very normal.
It's great, though, that your girl communicated her needs to you, and you listened. That's a good sign that you can talk openly about sex. That's really important. :)