Did you have them with milk? Were they softened and broken up?
If so it's very possible they just slid right through.
Gastric bypass leaves the stomach small but there's a opening on the side of it. Foods that are very wet and runny like soups just flow right through and there's no restriction.
Gastric sleeve and it's derivatives on the other hand keep your lower stomach sphincter. Your body is capable of holding more food and can stretch a bit more.
Now if you are 5 wheetbix dry, without milk. Then I'd say you should ask your team.
For example I'm also about 3 weeks RNY post-op. I just had some goldfish crackers I shouldn't have. But I ate maybe 8 or 10 of them and I can feel the pressure building in my lower chest. Once food is not able to go into the stomach and it sits in the esophagus, you will normally start to spasm. Cough and your will begin to salivate. It's incredibly uncomfortable.