I'm going to get pissed when people ask me about my weight
Why do you get upset when asked about your weight?
I feel like they're going to make a joke about me
Even if a questions bothers you, it's not polite to get angry at people. You can always decline to answer a question.
People shouldn't be able to freely question me about everything. Next time they question me I'll call them ugly.
Making fun of someone who asks you a question about your weight is rude. You never know if someone is going to make fun of you and its wrong to assume that they always are. Try remaining calm next time and just tell them politely that you don't wish to discuss your weight.