Something similar was happening to me last year around this time. I went to ER because I was in so much pain in my left leg (it literally felt like something was attacking my leg).. the doctor I saw there had lay down and stand and she checked my heart rate both times and gave me a referral to a cardiologist… I had to do an EKG, and echocardiogram, and a stress test.. and wear a heart monitor for 2 weeks.. cardiologist said everything was normal.. fast forward to this year I started having issues again and my rheumatologist suggested that I bring up POTS to my cardiologist… I didn’t even know what pots was until I researched it.. And just based off what I read I think last year was the beginning of me getting POTS .. now when I stand up my heart rate jumps really high and it stays high with just me standing.. I posted about it if you want to go and read it. I saw my cardiologist again last week and we discussed POTS and he did say it’s hard to diagnosis and definitely hard to treat but we can go that route and do testing.. so I have to do another echocardiogram and stress test today! And then possibly wear a monitor for 4 weeks. He also mentioned I would have to do a Tilt Table Test at out hospital after wearing the monitor..