Portuguese 7 days running 5-6 secs slower daily

I am a proud owner of the portuguese 7 days for six months. At first, it runs +4-5secs faster daily and I read that it's normal and have been enjoying and getting used to 'additional' time gained by the watch since then. Recently, I realized that it is running 5-6 secs slower daily, and there isn't any changes to my wearing habit or frequency of use. Is this a concern that I should bring back to the service center to check? I read that IWC watches are made not to run slower; is what I experienced abnormal?

A good question ... I have read that the 5001 calibre can run up to 15 seconds a day fast and that this was not unusual. Plus 5 seconds a day is still within chronometer standards and I `d be pretty happy with that with this model.
I too would be interested to see what others say about this.
Regards
Neil.

But mine is now running slower (5-6 secs daily) when it used to be plus 4-5 secs. That is what I puzzled me.

Your watch may have become slightly magnetized. This can easily happen if wearing it while passing through an airport metal detector or leaving it close to a laptop, TV, mobile phone or other electronic device that creates magnetic fields. It is very easily rectified at your IWC service centre. The watch won't even need to be opened.
Even if this is not the problem, as your watch is still under guarantee, IWC should regulate it back to within its operating tolerance of between -2 to +7 seconds a day without charge.
Leo