My butt feels hard on the side post-fat transfer. Is this normal?

When you have surgery to both of your buttocks, their recovery will impossibly totally symmetric.There will always a  difference between the two sides.You normally need to allow 4-6 weeks for this to settle completely.If firmness still exists after that time, one can start to consider other options, but at this stage you do not really need to be concerned.

Swelling after a BBL is normal and this may make the area feel firm. However, fat necrosis can also be very firm and no one here can be sure what is going on without an exam. You should see your plastic surgeon for an examination and to be sure. 

This firmness of your buttocks can be normal.  Swollen ankles can occur but you need to minimize this by elevating them when you are lying down.  The swelling needs to followed very closely!