Did you get any benefits in kind? Like private medical or anything? That's reported separately from your payslip and whilst your tax code should be adjusted to capture the tax on this it isn't foolproof so can cause tax payments.
Does it say what tax year you owe for? If it is the 20/21 year (ending 6 April 21), that's the one where tax returns are now being filed. It should be pretty easy to work out any underpayments for that one as you just had one employment and the year has ended. Shove your p60 amounts through a n online tax calculator and make sure they match.
What I suspect though is that given you have had multiple employments in the current tax year, this has caused a wrinkle somewhere. Definitely try and give them a call but what you could do is try and estimate what you should be paying:
April to when you left - p45 income amount
Next employment - p45 income amount
Next employment - p45 income amount
Current employment - take the taxable pay to date figure.
Add these all up to get your income so far. Then, if your income is roughly the same each month, add in an extra 3 months to get an estimate of what you'll earn for the whole of the tax year (ie what you'd expect to earn form jan-March).
Shove the total amount in one of the online tax calculators so it shows you what your annual bill will be. Then look at the tax you've paid (including an estimate for the 3 months of jan - March which you've estimated based on you payslip). That will give you an idea of whether there is a gap.
In my experience HMRC are usually pretty good at catching these things and whilst they can be wrong I would say it's probably sensible to assume they are not. They are usually pretty helpful if you talk to them though. If you haven't been paying enough through your tax code then it's better to get it corrected ASAP and is hopefully likely to be a one off issue due to the multiple employments this tax year.