Why is my rhr increasing?

Wondering if anyone has any thoughts on why my resting heart rate would be increasing? I have a Fitbit, and I was happy to see that since I started working out with FB again in November my hear rate had dropped from about 61-59 to an average of 55 (lowest 53). I was really happy at this little "bonus" benefit of working out. I thought for sure with adding more HIIT with the program I'm doing now (FB30 Round 3, as opposed to Low Impact and Flex, which I did in December and January respectively), my heart rate would continue to improve. This week however, it's been creeping steadily upwards and today I'm back at 59. Funny thing is, I've been sleeping and eating really well this week and have not had any alcohol. I'm not feeling extra stressed and am not taking any different medication. Just wondering if others have experienced this and if these increases may be temporary. Thanks.