Questions & Answers about Hoy me siento mal.
Spanish has two different verbs here:
sentir (non‑reflexive) = to feel / to regret / to sense something
- Lo siento. = I’m sorry.
- Siento frío. = I feel cold (I sense cold).
sentirse (reflexive) = to feel (a certain way, in yourself)
When you talk about how you yourself feel (physically or emotionally), you normally use the reflexive verb sentirse, so you need the reflexive pronoun me: me siento.