Hi, my name is DivThomson and I'm lookingoing for advice regarding backdated P.I.P payments

I have just been awarded P.I.P but 2 weeks later I've not received any correspondence regarding it. I worked it out I'm receiving the low rate p.I. p. I have been waiting a couple of months for my decision and the only reason I knew I'd been awarded p.I. p was because I went to the bank and there was the correct amount for 1 month and on my bank statement it said p.I. p . Should I not be entitled to any back payment considering I have been waiting a few months and then my claim was successful.  Any advice on this issue would be appreciated fully.David Thomson