It may be that the baby's bottom and back are against one side of your uterus (very common at this stage) so when you have a braxton hicks contraction the butt and back are pushed more tightly against that area and it feels really hard. After a while it can make that area quite tender to touch. When everything is relaxed can you feel your bub's back/head/butt? The butt and head can feel quite similar, lol. Don't be afraid to push quite hard on your tummy, you'll hurt yourself before you hurt bub. I love pushing bits of the baby around. I feel like I'm already interacting with the baby when I play with it's feet and arms. My huge tummy muscle separation makes it easier to feel body bits because there's one less layer to feel through