Sounds like the security guard was just a knob

That security guard should be ashamed!

(edit: Yes, and arrested for assault!)

If this was in the US, you didn't have to do that. The most they can do is detain you until the police arrive if they have probable cause under shopkeeper's privilege. They would have to be able to articulate which behavior rose to that level. It may make things easier to comply, but removing clothing is unreasonable.

don't be.