A Nice Day!

Today was a hot day,and the sun seemed to be bigger than before,which was what I thought.I am always working even today.

Since you said that today was a hot day. That means that today is over with and you are writing this in the past tense. In this situation "might be" is written in present tense. In order for it to agree with the rest of the current sentence, it would need to be written in past tense. The easiest way to do this is to use "seemed to" which is a synonym in the pst tense that goes with the rest of the sentence. In the second sentence you should say " I am always working, even today" If you say "WAS" and then say "even today" than it contradicts what you are trying to say. 

I tried to simplify the explanation as much as possible, I apologize if it does not make any sense and please forgive any typos... I type kinda fast and it can eventually lead to errors in my spelling and punctuation.