I'm upset that my best friend gifted me a mug for my birthday when we got her tickets for a weekend in london for her birthday.
Why do you think she gave you a mug?
Because she knew I liked the mug and I wanted to have them too.
I understand why you are upset.  However, you shouldn't get angry because it wasn't an expensive gift too.  You like the mug.
Why should I not be angry, she was rude and this is the last time I'll hang out with her.
It's not right to expect certain gifts back even if you gave something more expensive. Maybe she didn't have a lot of money to give.