My heart rate went weird

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Lol so here is what happened. I have been exercising every day around the same time. I do Leslie Sansone 2 mile brisk walk. I always wear a heart rate monitor and chest strap to monitor my HR. I always get my HR up to about 145 or 75% max heart rate. Today I did everything the same and before I even got out of the warm up my heart rate was already at 145, and maxed out at 165 or 85% at the most intense part of the exercise.

The only thing that changed was I started a new prescription for my thyroid, because I found out I have hypothyroidism. I took the pill last night before bed. I'm not worried about anything, but I am curious to know if it was the pill that got my heart going more? Thoughts?