I have two different things in my heart that will give me a higher BPM when I a) don't hydrate correctly and b) have lower electrolytes.

One is mitral valve prolapse and the other is pvc (pre ventricular contractions or complexes depending on who you talk to). The pvcs are just irritating as they feel like there is a marble stuck in your heart. When I don't drink enough water or my electrolytes are off, it acts up and usually shows up as a higher HRM when I exercise.

Don't forget your HR is based on age too.

Hope it is something simple