I work out 3-5 days a week for last two years, had to take a break last pregnancy. noticed my resting heart rate is decreasing. right now as i type this its 55 bpm. i feel normal. is lower heart rate because i'm more athletic and my heart is stronger?

If your workout is primarily aerobic activity such as jogging, bicycling etc, your heart rate may be a sign of conditioning and therefore good news. If you have had a recent examination and no abnormality was found, it would be safe to conclude that this is indeed the case. The heart rat alone does not necessarily mean that your heart is "stronger" than it was previously, but it seems likely.

It depends on your heart rate in the past. The heart rate is a balance between sympathetic and parasympathetic systems. If your parasympathetic system is stronger, your heart rate is usually in the low side. For athletes, the heart is remodeled with a high stroke volume and low heart rate (athlete's heart). If your heart rate was higher in the past you can now bring it down below 60 with training.