If you owe excess tax for 2014 or expect you'll be short on your tax liability in 2015, you will need to adjust your estimated tax payments now. If you wait until the end of the year, you just might be to late.
One good thing is that you need not pre-pay all the tax you might owe for 2015. By law, all you have to pay is 90% of the amount you think you owe or 100% of your 2014 tax liability, (110% if your AGI exceeds $150,000 in 2014). Doing so avoids the underpayment penalty,even if you end up owing the IRS a lot of tax.