Pip Change of Circumstances Backpay

Hi, I’m hoping someone can help me figure out how my backpay has been calculated - I requested a change of circumstances on 17th Feb as my conditions have become significantly worse, I was on standard daily living and have now been awarded enhanced for both daily living and mobility. I had my assessment on the 5th June and checked the gov website yesterday to see if it had been updated which it had so I haven’t received a text yet from pip nor have I received my letter.

I checked the amount of my next payment and it says £1134.20. My normal payments were paid around the end of each month so if I hadn’t done a change of circumstances, I would be expecting my next normal payment to be on the 26th June.

I’ve tried to calculate the backpay myself and was expecting more around £1800 ish going from the 17th Feb so I’m confused as to how the £1134.20 has been worked out as well as when I can expect to receive my new monthly payment. If anyone is good at working this stuff out I was hoping for some advice, I’m going to give pip a call tomorrow but I just want to get a bit of an idea before I speak to them, thank you in advance!