breastfeeding/bottle feeding

Laura
Hi, 
My little boy is 10weeks old today and has been exclusively breastfed until the past couple of nights where we have decided to do just bottle but breast through the day. 
However he has been a little fussy on the breast lately (which is why we decided to try the bottle at night) when he has a bottle he will guzzle a full 8/9oz no problem without throwing any back up and he sleeps a lot better.
I'm just wondering do people think it's time to switch to bottle with him drinking so much because I can only pump 4/5oz per breast?
So I'm worried he isn't getting enough from me to fill him completely .. his nappies and weight gain are fine .. just wondering if it's his way of telling me he wants more now? 
Sorry if it's a bit long winded but any advice would be appreciated? 
Thanks :)