BLUE TONGUE SKINKS

Hello everyone, I’m a little worried about my skinks behaviour. I got a Northern bts 5 days ago, he is 11 months old atm. When I placed him in the enclosure, he burrowed under the substrate and stayed there for 3 days. I decided to leave him alone so he can settle. Then, he came out and was quite active exploring the tank. However, I’ve noticed a weird movement he makes. It’s like he shudders sharply when he lays in basking spot (usually after a meal) It looks like a massive hiccup without sound. I’ve seen it at least 3 times. Otherwise he is being active and looks completely fine.
The temps are: basking spot 37-40 C, cool side is around 25C. Humidity is 50-60%

P.S sorry for any mistakes, English is not my first language