lifting heavy and sprinting should help with your heart, as both get it working really hard in short bursts. but, your heartbeat can quicken for a lot of reasons even while your resting, like any kind of inflamation or infection, or even to pump more blood to those areas affected by lifting and sprinting.
this seems like a perfect subject for a little experimentation. start measuring your heart rate at different times to see how it's affected: upon waking, after eating, post workout, way post workout, before falling asleep, etc., and see if it changes on workout days and rest days. i would also rest for full week, perhaps to get a sense of baseline. measure for a while, then increase your cardio for a few weeks and measure all the same times again. granted, it's only a n=1 experiment, but it would be great to see how cardio and lifting affect heart rate differently.