Is this all my fault??

Yes, a HR of 150 in a 15 year old should set off alarm bells all over the place, both literally and figuratively. Given the patient's history of Crohn's - likely on some form of immunosupressant therapy and therefore less likely to mount a febrile response - and a ruptured appy, I'd be very suspicious of sepsis and make sure I brought it to someone's attention - even if it's been documented as being communicated and even if the overall PEWS score hadn't changed. (I'm Canadian too, and our hospital has been using B-PEWS for probably 5 years.) I might preface my reporting of it with, "I'm perhaps way off in left field, but I'm concerned that this boy's HR was already high at 120 yesterday, and today it's persisting in the 150 range, with no other obvious changes. Is there something I'm missing?" Sometimes they're a little dry and some fluid brings it down. But in retrospect this boy was looking a bit shocky and there should have been more of an assessment done by all involved, not just you, the float nurse. Many people don't realize that what signals impending shock in adults isn't present in children until the very last minute. They maintain their cardiac output by driving up their HR - BP doesn't fall until very late in the process. Most 15 year-olds behave more like adults, but there are always those outliers.

As far as his discharge, deterioration and subsequent readmission being all your fault, it's not. There are other people in the chain of survival besides the nurse. Physicians are responsible for their own assessments; that isn't something they can legally delegate and if this boy was not assessed by a physician prior to discharge, that needs to be addressed. No peds physician worth the title would ignore that HR. The fact that you were floated out of your own area of expertise is also a system factor that can't be ignored. Yes, this situation gives the hospital a black eye, but it's a shared black eye so don't be so anxious to claim it! Learn from this and move on.

Like the comments above me said, 150 for a teenager is extremely high. I understand your thought that his trend had been in the 120s, but even that is high so I would have notified of the increase to 150. And if the charge nurse and physician would have been okay with it, I would have wanted them to explain their reasoning for why they think his HR was 150 if everything else was okay.

I have been using the PEWS system for almost my entire 10 year career and while I think it is a great guideline to use, it definitely isn't a black and white thing. I could have a PEWS of 1 but if I think something is wrong or off then I will absolutely escalate my concern as needed.

Another thing to consider is you stated you noticed his HR had been 120s but saw that it was charted that the MD and Charge nurse were notified - I would never base my decisions off of what was charted by previous nurses. I say that because you should always CYA, your license, and your patient - I can't tell you how many times I've looked at the previous nurse's charting and they've essentially copy & pasted assessments, have charted things that weren't there, or have missed huge things. So just because the charting says the MD and charge nurse were notified does not mean that they were notified.