Hot showers have a tendency to lower our blood pressure and to compensate for that our hearts speed up. To counteract that you can try and take cooler showers or make sure you are fully hydrated before you get in. If you can, check your pulse to see how fast it truly is going. But if you do not have any other distressing symptoms like shortness of breath, chest pain or feeling as though you will pass out and you rebound well once you get out of the shower then you are likely fine. That said, I am not a doctor and only go by personal experience so if you at all feel as though something is not right then just go and see your doctor to get a check up. Take care.
Do you feel this sensation at any other time or only in the shower? Any other symptoms such as dizziness etc?
You might consider taking a water proof watch into the shower with you. When you have an episode, measure your pulse rate to see if it is increasing or feels Irregular.