It can take some time for the IRS letters to make their way into the postal system and get to you. There isn't much you can do until you have the letter that explains what they need from you.
However, if you are 100% sure that she said it was flagged as a frivolous return you need to start looking for a professional near you, preferably a tax attorney. Frivolous returns carry steep penalties ($5,000 in addition to accuracy-related and civil fraud penalties) and are one of the few things in IRS parlance that equal criminal charges.
I have never heard of an agent saying this before, which is not to say it didn't happen to you. I just hope that what she actually said was something other than frivolous or that she misspoke.
MOST of the time the hold-up is an extra layer of protection for identity (in which case you often need to do nothing), or a matching issue in the systems which just require a human at the IRS to resolve the issue and move the return along.
Frivolous returns, though, are a very serious charge - so if that is what your letter indicates when you receive it, please do not respond on your own. Reach out to a qualified professional near you.
The most common type of frivolous return is the "tax protestor" - if you filed a return with the position that you don't owe tax because it's unconstitutional or some similar nonsensical argument, that is when frivolous might apply to you. it would be hard to have this designation in the course of an ordinary tax filing. If you didn't take a position like this or something else that is not "typical", then this is highly unlikely to be the real reason for the review.
