There are many reasons why your heart rate might be higher than normal. One is dehydration, mine is much higher whenever I am dehydrated...my resting is fairly high to begin with, around 100 to 110, it doesn't take much to make my pulse jump but being dry does not help. Two is infection, whenever I have an infection, my heart rate jumps very high and my SVT (supraventricular tachycardia) acts up nearly every time, my heart rate has been sustained in the 165-170's range at times. Very uncomfortable.
I also am thinking perhaps your thyroid medication could be too high. Too much synthroid will cause your heart rate to go higher than normal as well. Our CF meds tend to cause a high heart rate as well, especially the albuterol treatments. Low O2 levels will also cause your heart to work harder, thus increasing your pulse.
As you can see, there are many reasons why your heart rate could be sitting in the 120's range. I hope you feel better and that your heart behaves itself, you sure don't need another health issue to contend with..CF is complicated enough!
Hugs, Jenn