I don't feel well.

I had an awful headache when I got up this morning, and I have not been feeling well all day long, so I am a little worried. I think that I need to go to the doctor before going to work tomorrow.
So With that being said, the appointment we had agreed on it will not be possible; however, we can go out on Saturday.

Aside from some misspellings, there wasn't anything that was wrong, but some things were awkward, so I changed the syntax for you. Also, in English, you're not supposed to start sentences with coordinating conjunctions. You can (and often do) in speech, but for more formal writing contexts, it is frowned upon; however, ending clauses with prepositions is considered poor grammar as well, but I've already done it twice lol Neither rules are observed very strictly anymore, but I'm a grammar nazi when I'm learning a new language, so I correct people as I would want to be corrected. It was totally comprehensible, though!!

I don't feel well.

I had a strong headache when I got up this morning and I have not been feeling well all day, so I am a little worried. I think that tomorrow before going to my job I will visit the doctor.
But the appointment we had agreed on will not posible, so we changed it for Satuday.