Could someone tell me if this is health anxiety related please?

Hi,

For the past 5 to 6 days iv noticed that if ive sat/lay a little to long (watching tv, playing a computer game, or with my kids) when i stand up my heart starts to beat rapidly, also at the top of my chest/lower neck area there seems to be a "Flutter" almost like i can feel my heart beating thru my chest!

The past couple of times ive been in the bath the same thing has happened. It leaves me with shortness of breath, the rapid heart rate does settle down if i take deep breaths but it really does feel like my heart is pounding like ive run a marathon or something.

Id really like to know if this is health anxiety or maybe general anxiety, Maybe its because im sitting a little to long, but why would i be getting it in the bath? I know i have anxiety, im currently on 20mg Citalopram. I had a full ECG last year and the results were normal. This is really worrying me now.

Thank you all in advance!