Can't add spells to my spell bar.

Yesterday when I was trying to catch a mount I realized I didn't put mount taming on my spell bar, so I thought ok no worries pull up the spells list and add it on. Well, it wouldn't let me add it to my spell bar. Any spell couldn't be added to my spell bar during the fight, even my class spells. I tried clearing my cache mid fight, didn't work, and even in a fight afterwards I tried it again to see if it's a continuous thing it wouldn't let me add spells to my spell bar. Outside of fights it works fine and normally, just in fights you can't add a spell to the spell bar anymore.