It is not true that 'almost all westerners' think this. 'Hot' is the opposite of cold. Among English speakers, the only people who would pretend to misunderstand 'I'm so hot' are those who .... have an immature sense of humour ... are overly influenced by American teen media You could avoid the issue by using the impersonal construction with 'It is', if you wish, by saying 'It's so hot here/today/now', but it isn't necessary.

I think it is not 100% true But may I have a fever Will be better?

You can't avoid the people that pretend to not understand the meaning of this, but everyone does understand it. This is how you correctly express that you're not cold, you're hot, and the majority (almost all) of Westerners use this expression.

it's hot

Well, that's a good one. lily