"I felt tired today as i worked hard yesterday." is fine.
Correct usage of
"I felt tired because of i worked hard yesterday"
would be :
I am feeling tired today because I worked very hard yesterday.
I would like to emphasize that "because of" won't be a substitute of "AS".
You can use the following for "AS" :
SINCE, BECAUSE , FOR.
These differ based on the information you already have :
AS, SINCE generally come in the beginning of sentences.
Also, they are used when the reason is already known to the listener/reader, or when it is not the most important part of the sentence.
As he wasn't ready, we went without him.
Since she wanted to pass her exam, she decided to study well.
BECAUSE and FOR come in the middle of the sentences as conjunctions :
BECAUSE often introduces new information which is not known to the listener/reader. It puts more emphasis on the reason.
We use a FOR-clause when we introduce new information. A for-clause often expresses an inference.
I read because I like reading.
I decided to consult a doctor for I was feeling bad.