Could this actually be my MAX HEART RATE?

Not a doctor but have done some HR based training in my time. 254 is not normal in my book, at any age, at any intensity. My first guess is that your heart rate monitor is reading incorrectly. I'm not familiar with that particular model but most models have their quirks. One I used to use had the habit of suddenly declaring me clinically dead (HR=0). It sometimes insisted my HR was 110, even when running hard, uphill. Another at the gym (on a stationary bike) sometimes tells me my HR spikes to 220, then suddenly drops to 90. I've checked with my own monitor and it's been in the 150's the whole time! Common causes would be the chest strap being too loose or improperly positioned,  contact points not being wet enough, extreme temperatures, failing batteries, etc.

Is pushing the pace/effort that hard something you often do in training? Or something you specifically did to try to find your max HR?  How were you feeling when you supposedly hit 254? If there were any symptoms like chest pain, shortness of breath (beyond what's typical on a hard training effort), dizziness, etc. then definitely you need to see a doctor ASAP. If not, I'd suspect the monitor, although it wouldn't hurt to check things out to make sure all systems are OK, if only for peace of mind for you and your family. In any case, there's no need to do an all-out effort to find max HR.  What you want (assuming you're healthy) is to do an LT test, which I'm sure you can find info for on this site and elsewhere (for running, roughly, your average HR for a hard effort that you can sustain for the last 20 minutes of a 30-minute run). That yieleds much more useful info for calculating HR zones for training.

x2  Try checking your heart rate manually right after a hard workput to see how close it is to the Garmin.  My guess the monitor is off.  My wife has a heart condition that causes her HR to rise rapidly at times.  When hers gets over 200, it is so fast and weak, I have difficulty getting a reading.