The bars on the IRS.gov's "Where's My Refund" webpage (which it is believed you are asking about) change all of the time. There's no reason to be especially concerned, or to necessarily think that your tax return is under review. If your return is actually chosen for further examination, either at random or for a specific reason, then the website should clearly state that, and also let you know to expect a forthcoming IRS notice by mail. (You haven't observed that happen, have you?)
Taxpayers who do have their returns held for review, and who expect to receive an IRS notice, tell us that such a message appears clearly on the Where's My Refund website. The behavior that you observe in the graphical bars is more likely an indicator that your tax return is simply moving through the processing system. It could also be that the IRS is checking for additional information on something related to your tax return (like matching your reported W-2 data with that separately reported to the IRS by your employer); but we wouldn't call that a review . . . perhaps only a small delay.
If enough time passes after filing (21 days after e-file, or 6 weeks after paper file), and there is no new information, at that time the IRS indicates that you can call them for additional information, using the phone number provided on their Where's My Refund website.
For now, the best course of action is patience. The IRS doesn't always move as fast as taxpayers would like (especially when waiting for a refund).
Thank you for asking this question.
