Who knows with so little info. See a Dr. Self-diagnosis over a period of time is beyond foolhardy!

Riding route changed or modified?
Develop an arrythmia? Bundle branch block?
Time of day change for your rides?
Temperature changes?
Stress? BP changes?
Resting heart rate change?
Did you suddenly get an aero position?
Illnesses? Inflamation?
Other activities?

Equipment wears out. Double check with new gear.

edit:
My guess is = you went tubeless and the lower rolling resistance is immediately obvious in effort required.