The traditional aneurysm forms as a result of high blood pressure weakening the vessel wall -- which is a process that generally takes years to develop.
However, there are some cases where an existing aneurysm can suddenly enlarge over the course of days or weeks. But for this to happen, the aneurysm usually has to already exist, and it's likely that this was present for months to years prior to the rapid enlargement.
Given that you have had normal studies, it would be very unlikely that you would develop an aneurysm over days to weeks from now (though not impossible).
Honestly, it is difficult enough to treat headaches after evaluating you in person -- it would be impossible to say anything over the Internet that could really be applicable to you.
Headaches are due to a number of different causes -- but luckily with the clear MRI/MRA, some of the most concerning causes (ruptured aneurysm, tumors, increased intracranial pressure) have been ruled out.
Your best bet would be to make an appointment with a neurologist who specializes in headaches to evaluate you.