I bet if you co sleep, the backache will fade away. If it has come on since the birth, it is probably a mixture of stress (mental) and physical strain from sitting feeding. If you co-sleep, facing your baby, your spine will be in a neutral position all night and the discomfort will probably fade away.
I have got to admit, I did not want to co-sleep with a young baby, due to safety implications. I read 'the science of parenting' by Margot Sunderland and looked up safe co-sleeping on the internet. The main points which increase danger are:
Drinking alcohol - a big no no you must abstain.
Baby sleeping between two adults or with other young children in the bed - I kicked my husband out till the baby was 1 year old, he does shifts anyway so was kicked out when I was up all night sitting feeding as well to ensure at least he got a good nights kip.
Bedding - to decrease the risk of suffocation, remove any heavy bedding. I decided to wear really warm pj's and a cardigan with a small fleece blanket upto my waist. The baby had sleep suit and vest with one small baby crochet cover (we had heating on so not cold at all). You could sew pillow cases to the bed sheet so they cannot move, or go without like I did until baby was one year old!! ( I really did my best to make it as safe as possible!)
Over tired parent - you should avoid co-sleeping when you are extremely tired (well, after a few good nights kip - this will go away!)
Overweight parents - the risks in co-sleeping increase if parents are overweight.
Falling from bed - falling and injury from fall or falling down between bed and furniture etc. Simple - move furniture and have bed in middle of room. You could put down a mat or bedding on the floor to reduce risk of injury in unlikely event baby falls.(this is what I did) Or I suppose you could buy a bed guard.
Please look up on line as it is likely I have forgotten something as it was such a long time ago now.
Also, consider your sleep style. I sleep in the same position every night on my side and wake up to turn over. I do not move in my sleep and wake up very easily so I considered that co-sleeping, adhering to all the above, was safer than sitting up all night with the possibility of falling asleep whilst sitting and baby going who knows where.
Beware, a lot of people oppose co-sleeping. They must never have been in the position you are in where you have no choice. I chose to keep it to myself and a few choice relatives who I knew would be supportive.