I have just re connected my bank account, and after doing so my income / expenses have gone way done , as it has back dated

Hello, harryfearnall94-. 

Glad to see you here in the Community. In QuickBooks when connecting the bank up it only brings in 90 days worth of transactions, if you need more than 90 days you will then need to CVS the rest of the transactions into your account. Please refer to this link to do import older transactions manually. 

Then, you'll have to categorise the transactions. 

I want to make sure everything is taken care of for you, please let me know how it goes or if you have any other issues or concerns.