You may have to wait longer. The yawning actions they do after swallowing prey are part of working the food down into their stomach. Wait until the yawning has stopped and then offer more food. Hopefully this doesn't keep occurring.
I was wondering why they yawn slightly for a minute or so after they eat something. Cool to know its part of their swallowing mechanism. I knows frog buddy won't take another food item for about a minute so I don't even try till he's finished yawning.