Hi all x silly question but

I've really struggling with my heart rate. In the last few months I've gotten it down to a health 75 bpm which is good for me. Yet since last week it's been racing again, I haven't been nervous, sick or anything. From just knowing from testing it all the time in the pass it's been stuck at around 120 at rest and up to 140 ish while you and about. Today I was at the airport and I could truly feel it harming out of my chest- so the first time in forever I checked it. It was 184 bpm. I've been feeling rather short of breath and phycial exhausted also. It's come down to about 130 ish maybe Abit higher since but I feel very on edge on :/ I've been to the doctor before about my heart and she said it was all fine but I don't know how this can be fine