When does myocarditis occur after a cold?

Hello,

I have a question. My boyfriend has had a cold since last week. He didn't have a fever, just a sore throat and the usual. We were away over the weekend, though. We took a short vacation over the weekend and did quite a bit of exercise. We were out and about continuously from Friday to Sunday. It was okay for him, too.

We checked his heart rate yesterday and saw that at times while walking, his heart rate was over 140 bpm. He also said he was a bit tense during the walk, and his heart rate might have been so high due to his excitement.

Today he is feeling better, but he is still a little under the weather.

Now I'm a little worried about him. What if his high heart rate could cause him to develop myocarditis? Is that possible? We haven't been doing any exercise in the sense of that. Could he already have myocarditis if he's only been sick since last Thursday? Or does the inflammation take longer to develop? When would the first symptoms appear? He doesn't seem to be really worried about it yet, which is probably for the better. But he's generally more relaxed. So I want to keep an eye out for warning signs and need your help.

Thank you!