You don't say who took your heart rate.
But like blood pressure, heart rate should be taken multiple times to be sure the one reading is an accurate reflection of your rate.
It is easy to take your own pulse.
Check it right now and at least several more more time to be sure that low rate of 45 is really an accurate reflection of your heart rate.
[An easy way is to check your pulse at your wrist over 15 seconds, then double it and double it again to get your rate for one minute.]
Is your pulse [heart rate] still at 45?
El Diablo is right that a low heart rate could be reason for medical concern. Could be. Not necessarily so.
So there is no reason for you to be worried over something like this - if indeed your heart rate is this slow on multiple determinations do the smart thing and ask your doctor. No one on Talk Tennis or any other internet site can tell what is going on in YOU. Why not just get checked out and find out for sure?