What Is The Cause Of Changing Heart Rate?

Hi, I have a question about recovery heart rate. I find this a bit confusing. I am a 33 year old male who only does weighttraining, so no cardio . I m actually affraid to do cardio. My question is this. When I get my heartrate up and make it stay there for a short period of time (2-5minutes), then my recovery heart rate is great. I get my heartrate up to 150or160, and after one minute it s around 120 to drop below 100 after two minutes. BUT, when I do cardio for 30 minutes (which I almost never do), and I measure my heartrate afterwards, it s also 150, but it doesn t go down the first minute. So my recovery heart rate is terrible after long sessions. It starts slowing down after a couple of minutes, but this goes very slow. It returns back to normal after about an hour. With normal I mean around ten beats higher than my resting pulse, which is 50-55. Is this because there s something wrong with my heart, or is it because my body is trained for short outbursts and not endurance. Thx in advance p.s. I ve had several ekg s, thorax photo s, blood tests. All came back normal.