Questions I’m too shy to ask my mom

Hi everyone! So I’ve always wondered these things but I’m a very personal and shy girl, so I’ve been too scared to ask anyone I know. Please help me out :)

I’m almost 18, been masturbating since i was 12, never been touched by someone else. I have always been more comfortable rubbing my clit, instead of fingering myself. It feels good at first, but then After about a minute, my legs start to shake and then the pleasure goes away. It doesn’t turn to pain, it just disappears. Here are my questions:

1. How can I tell if I’ve orgasmed?

2. If what I experience isn’t an orgasm, what is it?

3. Is it normal to only ‘last’ for such a short period of time?