Get a good nursing pillow, it really helped me. Also do some feeds laying on your side maybe to give yourself a break.
Everything Sadie said, and also side-lying position during nighttime feeds helps too. Be careful how you are moving baby when you're in bed. I was using my back too much to get her in and out of bedside bassinet. Now I try to use my arm muscles more. Also, when I turn over with her, I put her to my chest and log roll. It also gets better as your muscles get stronger, you'll get there.
How are you holding baby up? I make sure I have a comfy position on supportive pillows and I use my pregnancy pillow as a nursing pillow in my arm so I’m not using any additional effort to hold him up