Relaxer and heat damage ??
MY hair was last relaxed in 2009. AFTER 2009, I kept applying heat. NOW, IM transitioning, my top is 3b and my bottom 2b ... HOW do i get my curls back on the bottom half ! MY hair is more healthy now then before. PLEASE HELP ME !
The only ways to fix it are: chop it off or 'transition' by blending the 2 textures with bantu/twist/braid outs. I had the same problem a couple years back and I wore a bun until I had enough length on my healthy curls to cut the straight bits off.
Sounds like heat damage. Just be extremely gentle with your hair/ends and get regular trims until the damaged hair is gone. Good luck!