Just wanted to say my DD was the same way. Up until about 5 months she would pretty much ONLY nap on me while nursing. She also did not like to be worn while I was sitting (still doesn't...). What I ended up doing was keeping a pillow near the computer to put under her & padding my elbow from the hard plastic arm of our computer chair (I have tendonitis so that was a BIG issue for me). Luckily our computer chair already has a pretty cushy seat. :LOL
The biggest thing that helped was just her getting older. At almost 5 months she hit a huge developmental spurt and really "woke up" to the world around her. Now at almost 9 months I have the opposite problem that she NEVER wants to nap on me!
: She's much too busy to lay in Mommy's lap all day.
The only other suggestion I can offer to to try transitioning to naps alone on the bed. I started by swaddling her (but she always starts out sleeping swaddled, so you may not have to do that part), putting on some white noise, darkening the room, and laying down next to her to nurse. Once she's out I'd *very* slowly and gently ease my nipple from her mouth, and then slowly sneak off the bed & out of the room. This was NOT an overnight fix though. It took a couple weeks of doing this before she was able to sleep for any length of time alone on our bed. Gradually her time increased though and I was able to do a few things while she napped.