What equipment are you using? Garmin chest strap and snapped-on transmitter? Or Garmin strap with built-in transmitter? Garmin watch? Very good chance it's the strap or the transmitter's connection to the strap. When that happens, stop. If you have the snap-on transmitter, unsnap it, lick the connections, resnap. Either strap type, lick the rubber connection pads on the strap. That should fix it. As mentioned above, there's also a chance of jersey flap, which is a static electricity issue. Either way, your HR probably goes back to normal as soon as you stop. So do as above and get back to us.

Another possibility, if doing the above doesn't fix it, is that the electrical goodies inside the strap have gone bum. Chest straps only last for a few years, sometimes only 1 year, depending the the strap quality. I always have a spare strap for this reason. If you have the snap-on transmitter type, an $18 Polar strap works fine and can be purchased separately, as can the slightly nicer Polar strap for ~$34. If you have the built-in Garmin strap transmitter, buy the new snap-on Garmin type. All this available on Amazon.

OTOH, if you have some sort of wrist unit that doesn't use a chest strap, well, that's what you bought. Should have gotten the chest strap type, which also can have problems, but is usually much more reliable.

I don't mention the possibility that it could be your heart that's gone nuts, because that's not the problem. It's your equipment.