It may be that your employer didn't take enough tax on your salary, or you've got a new taxable item on a P11D that hasn't been reflected on a tax code.
You are given a tax code each year, which is used by HMRC to try and collect an estimate of what your tax will be for the year.
If your only income comes from employment income this is usually pretty accurate as the tax code is based on the tax free personal allowance.
If you were self employed last year, they may have attempted to adapt your tax code to collect the expected tax based on your income from last year. Obviously both salaries and taxable self-employed profits can go up or down, so it really is a best guess on their part.
To calculate the tax on your salary alone, take the total employment income an deduct 12,570. Let's use £43k as an example - £43,000 - £12,570 = £30,430. This is all within the 20% tax bracket, so £30,430 x 20% = £6,086.
Now, compare that to what tax you've had deducted from your salary, per your P60. If you've underpaid on your salary, this will be added to the 20% you're also due to pay on your self employed income.
If you still aren't sure, post accurate figures here from the tax calculation and I'll take a detailed look.