Nationwide Mortgage arrears statement?

Help!

I've just got in from work to a Nationwide Mortgage arrears statement, and now it's gone 8pm and I can't call anyone who can help explain what it's all about, and could do with some reassurance as my partner probably won't sleep now!

We pay direct debit on the 25th of each month, never missed a payment since we started in 2011 and enough money in the account to twice cover it just in case.

This letter shows that payments were made in jan & feb as normal but dated 3/3/13 we are 'in arrears' as we haven't paid march (but we won't have done as it won't go until the 25th) and its quite a official letter with a 'problems paying your mortgage' flyer included.

It says we must pay in 7days,

Now, this is the first time we have ever received this letter and nothing has changed, is this a (scary) admin error or something else? I've double checked all the payments went and the statement confirms this.

Why would they be asking for it outside our usual direct debit date?

Please help put minds at rest at least until the morning!

Ollie