I don't feel well
A very common and direct alternative, literally meaning 'I feel bad'.
Use this to state 'I am sick'. Use 'enfermo' if you are male, 'enferma' if you are female.
A more specific phrase for 'I'm a little dizzy' or 'I feel a bit nauseous'. Use 'mareado' for male, 'mareada' for female.
I'm not going to the party. I don't feel well.
Excuse me, can I go home? I feel bad.
I need a doctor. I am very sick.
I think I'm not going to eat that, I'm a little dizzy.
The phrases 'No me siento bien' and 'Me siento mal' are universally understood and used across the entire Spanish-speaking world.
While 'No me siento bien' is common, you will also frequently hear 'No me encuentro bien' (I don't find myself well), which is used in the same context.
It's common to use diminutives to soften the statement. You might hear 'Estoy maluquito/a' to mean you feel just a little bit unwell.
In addition to the standard phrases, you might casually hear 'Ando malo/a' (I'm walking around bad), which means the same as 'I feel sick'.
Saying 'No siento bien' instead of 'No me siento bien'. — The verb for 'to feel' (about oneself) is 'sentirse', which is reflexive. You must include the reflexive pronoun 'me' (myself). 'Siento' by itself means 'I feel' something external, like 'Siento el calor' (I feel the heat).
Saying 'Soy enfermo' instead of 'Estoy enfermo'. — Feeling sick is a temporary state, not a permanent characteristic. Use the verb 'estar' (to be) for temporary conditions, health, and location. Use 'ser' (to be) for identity, profession, or inherent qualities.
Using 'malo' instead of 'mal' in 'Me siento mal'. — 'Mal' is an adverb that means 'badly' or 'unwell'. 'Malo' is an adjective meaning 'bad' or 'evil'. 'Me siento malo' can be misinterpreted as 'I feel like an evil person'. The correct phrase is 'Me siento mal'.