Pip Backdated from Start of Claim

I applied for Pip on 18th January 2019 I had my assessment on 2nd May 2019 it’s normally 12 to 14 week including assessment putting on the claim etc - I received my money today from PIP just under £1,100 which works out roughly 13 weeks money - I have not got the decision letter yet so no idea what rate I am on, if they owe further money do think they will pay that separately because it looks like they’ve paid me from March onwards only is this normal if they gone outside the time frame they are 6 weeks outside the time frame - what do you think - or do you think best to wait to get my letter and it shall explain all it’s all new to me.