In English that is more formal, you never use it. Basic does not fit that context in formal English. I have never heard of it in my life - even as a slang or colloquial expression - it is obviously localized to a particular place.

It doesn't mean upset. It could mean you feel bored, but it could also mean you feel a little "flat" - that's to say, you feel a little drained energy-wise or emotional-wise. Similar expressions would be "I feel flat, I feel a bit average, I feel a bit so-so" These give the same meaning to me as "I feel basic". I think to understand the exact meaning you need to have a context in which it is used. I don't think there is one meaning to it. It's very, very rarely used where I am.

As slang, basic refers to someone who is unwilling or afraid to express individuality. If you are basic, then you are ultra-conformist, not unique, and status-seeking through group association. You are not offensive and may be a little boring.

A synonym for basic is easy. In this context I would suggest that it means boring.