Thought I was gonna die on the treadmill

I had it set for 30 minutes and was walking at 2.5 mph. My heart rate averaged about 135 to 140 bpm. Normal right. Well at one point about 20 minutes in I had a big heart palpitation and it felt like I had a lump go up in my throat. My heart rate shot to 150. I had to dummy down my speed to a crawl and get that down because that big ass palpitation did not feel good.