Step 1: Understand the meaning of "working overtime".
The sentence is in the past continuous tense, which is incorrect for the given time frame. The correct tense to use here is the present perfect continuous tense, which expresses an action that started in the past and continues into the present.
Step 2: Evaluate the alternatives.
"WORKING" is not appropriate because it doesn't express the continuity of the action over the last few weeks.
"IS BEING WORKING" is grammatically incorrect.
"HAS BEEN WORKING" is the correct form as it is the present perfect continuous tense, which is used to express an action that has been ongoing until now.
Step 3: Conclusion.
The correct alternative is \textit{has been working} as it conveys the ongoing nature of the work over the past few weeks.