I think consent was the issue. Ask her if she’s into it first, if she is, go for it! If not, maybe dirty talking might be more her style. Just ask :)

How hard did you hit her? You should start off light and let them say, "Harder, sir."

Either you slapped her WAY too hard or she was so startled when you slapped her, her crying was instinctive. Either way, apologize to her and ask her what she would like during sex.

Spicing things up is ok but you and her should talk about what things can be done this way neither of you are doing something that your uncomfortable doing. As for you smacking her butt and her crying maybe you hit her harder then you meant to and she got hurt.

What makes you think you can strike a female with no prior consent? My ex did that. I call it assault.

Too hard? without consent? yep that's assault

You reminded her of her dad and uncles

When you say spank, do you mean a hard hit? If so then that is something you really need to talk about before doing it.

You either hit her way too damn hard, or you brought to memory something from her past.

Spanking is really something you need to be sure your partner is into before doing. Consent is the key. Just apologise and ask her what she likes most in bed